Infectious Diseases HARR

Question 1

Which serum antibody response usually
characterizes the primary (early) stage of syphilis?
A. Antibodies against syphilis are undetectable
B. Detected 1–3 weeks after appearance of the
primary chancre
C. Detected in 50% of cases before the primary
chancre disappears
D. Detected within 2 weeks after infection

Answer 1

Detected 1–3 weeks after appearance of the
primary chancre

During the primary stage of syphilis, about 90% of patients develop antibodies between 1 and 3 weeks after the appearance of the primary chancre.

Question 2

What substance is detected by the rapid plasma reagin (RPR) and Venereal Disease Research Laboratory (VDRL) tests for syphilis?
A. Cardiolipin
B. Anticardiolipin antibody
C. Anti-T. pallidum antibody
D. Treponema pallidum

Answer 2

Anticardiolipin antibody

Reagin is the name for a nontreponemal antibody that appears in the serum of syphilis-infected persons and is detected by the RPR and VDRL assays. Reagin reacts with cardiolipin, a lipid-rich extract of beef heart and other animal tissues

Question 3

What type of antigen is used in the RPR card test?
A. Live treponemal organisms
B. Killed suspension of treponemal organisms
C. Cardiolipin
D. Tanned sheep cells

Answer 3

Cardiolipin

Cardiolipin is extracted from animal tissues, such as beef hearts, and attached to carbon particles. In the presence of reagin, the particles will agglutinate.

Question 4

Which of the following is the most sensitive test to detect congenital syphilis?
A. VDRL
B. RPR
C. Microhemagglutinin test for T. pallidum
(MHA-TP)
D. Polymerase chain reaction (PCR)

Answer 4

Polymerase chain reaction (PCR)

The PCR will amplify a very small amount of DNA from T. pallidum and allow for detection of the organism in the infant. Antibody tests such as VDRL and RPR may detect maternal antibody only, not indicating if the infant has been infected.

Question 5

A biological false-positive reaction is least likely
with which test for syphilis?
A. VDRL
B. Fluorescent T. pallidum antibody absorption test (FTA-ABS)
C. RPR
D. All are equally likely to detect a false-positive result

Answer 5

Fluorescent T. pallidum antibody absorption test (FTA-ABS)

The FTA-ABS test is more specific for T. pallidum than nontreponemal tests such as the VDRL and RPR and would be least likely to detect a biological false-positive result. The FTA-ABS test uses heat-inactivated serum that has been absorbed with the Reiter strain of T. pallidum to remove nonspecific antibodies. Nontreponemal tests have a biological false-positive rate of 1%–10%, depending upon the patient population tested. False-positive findings are caused commonly by infectious
mononucleosis (IM), SLE, viral hepatitis, and human immunodeficiency virus (HIV) infection.

Question 6

A 12-year old girl has symptoms of fatigue and a localized lymphadenopathy. Laboratory tests reveal a peripheral blood lymphocytosis, a positive RPR, and a positive spot test for IM. What test should be performed next?
A. HIV test by ELISA
B. VDRL
C. Epstein–Barr virus (EBV) specific antigen test
D. Treponema pallidum particle agglutination
(TP-PA) test

Answer 6

Treponema pallidum particle agglutination
(TP-PA) test

The patient’s symptoms are nonspecific and could be attributed to many potential causes. However, the patient’s age, lymphocytosis, and serological results point to infectious mononucleosis (IM). The rapid spot test for antibodies seen in IM is highly specific. The EBV-specific antigen test is more sensitive but is unnecessary when the spot test is positive. HIV infection is uncommon at this age and is
often associated with generalized lymphadenopathy and a normal or reduced total lymphocyte count. IM antibodies are commonly implicated as a cause of biological false-positive nontreponemal tests for syphilis. Therefore, a treponemal test for syphilis should be performed to document this phenomenon in this case

Question 7

Which test is most likely to be positive in the
tertiary stage of syphilis?
A. FTA-ABS
B. RPR
C. VDRL
D. Reagin screen test (RST)

Answer 7

FTA-ABS

The FTA-ABS or one of the treponemal tests is more likely to be positive than a nontreponemal test in the tertiary stage of syphilis. In some cases, systemic lesions have subsided by the tertiary stage and the nontreponemal tests become seronegative. Although the FTA-ABS is the most sensitive test for tertiary syphilis, it will be positive in both treated and untreated cases.

Question 8

What is the most likely interpretation of the
following syphilis serological results?
RPR: reactive; VDRL: reactive; MHA-TP: nonreactive
A. Neurosyphilis
B. Secondary syphilis
C. Syphilis that has been successfully treated
D. Biological false positive

Answer 8

Biological false positive

A positive reaction with nontreponemal antigen and a negative reaction with a treponemal antigen is most likely caused by a biological false-positive nontreponemal test.

Question 9

Which specimen is the sample of choice to
evaluate latent or tertiary syphilis?
A. Serum sample
B. Chancre fluid
C. CSF
D. Joint fluid

Answer 9

CSF

Latent syphilis usually begins after the second year of untreated infection. In some cases, the serological tests become negative. However, if neurosyphilis is present, cerebrospinal fluid serology will be positive and the CSF will display increased protein and pleocytosis characteristic of central nervous system infection.

Question 10

Interpret the following quantitative RPR test
results.
RPR titer: weakly reactive 1:8; reactive 1:8–1:64
A. Excess antibody, prozone effect
B. Excess antigen, postzone effect
C. Equivalence of antigen and antibody
D. Impossible to interpret; testing error
Immunology/Correlate laboratory data with

Answer 10

Excess antibody, prozone effect

This patient may be in the secondary stage of syphilis and is producing large amounts of antibody to T. pallidum sufficient to cause antibody excess in the test. The test became strongly reactive only after the antibody was diluted.

Question 11

Tests to identify infection with HIV fall into
which three general classification types of tests?
A. Tissue culture, antigen, and antibody tests
B. Tests for antigens, antibodies, and nucleic acid
C. DNA probe, DNA amplification, and Western
blot tests
D. ELISA, Western blot, and Southern blot tests

Answer 11

Tests for antigens, antibodies, and nucleic acid

Two common methods for detecting antibodies to HIV are the ELISA and Western blot tests. Two common methods for detecting HIV antigens are ELISA and immunofluorescence. Two common methods for detecting HIV genes are the Southern blot test and DNA amplification using the polymerase chain reaction to detect viral nucleic acid in infected lymphocytes.

Question 12

Which tests are considered screening tests
for HIV?
A. ELISA, 4th generation, and rapid antibody tests
B. Immunofluorescence, Western blot,
radioimmuno-precipitation assay
C. Culture, antigen capture assay, DNA
amplification
D. Reverse transcriptase and messenger RNA
(mRNA) assay

Answer 12

ELISA, 4th generation, and rapid antibody tests

A ELISA, rapid antibody tests, as well as the 4th generation automated antigen/antibody combination assays are screening tests for HIV. The 4th generation assays detect both antigen and antibody.

Question 13

Which tests are considered confirmatory tests
for HIV?
A. ELISA and rapid antibody tests
B. Western blot test, HIC-1,2 differentiation assays, and polymerase chain reaction
C. Culture, antigen capture assay, polymerase chain reaction
D. Reverse transcriptase and mRNA assay

Answer 13

Western blot test, HIC-1,2 differentiation assays, and polymerase chain reaction

Western blot, and PCR tests are generally used as confirmatory tests for HIV. An HIV-1,2 differentiation assay is recommended as the confirming procedure following a reactive 4th generation HIV assay. PCR, however, is more often used for early detection of HIV infection, for documenting infant HIV infection, and for following antiviral therapy.

Question 14

Which is most likely a positive Western blot result for infection with HIV?
A. Band at p24
B. Band at gp60
C. Bands at p24 and p31
D. Bands at p24 and gp120

Answer 14

Bands at p24 and gp120

To be considered positive by Western blot testing, bands must be found for at least two of the following three HIV proteins: gp41, p24, and gp120 or 160. The p24 band denotes antibody to a gag protein. The gp160 is the precursor protein from which gp120 and gp41 are made; these are env proteins.

Question 15

A woman who has had five pregnancies
subsequently tests positive for HIV by Western
blot. What is the most likely reason for this result?
A. Possible cross-reaction with herpes or EBV
antibodies
B. Interference from medication
C. Cross-reaction with HLA antigens in the antigen preparation
D. Possible technical error

Answer 15

Cross-reaction with HLA antigens in the antigen preparation

Multiparous women often have HLA antibodies. The Western blot antigens are derived from HIV grown in human cell lines having HLA antigens. A cross reaction with HLA antigen(s) in the Western blot could have occurred.

Question 16

Interpret the following results for HIV infection.
ELISA: positive; repeat ELISA: negative; Western blot: no bands
A. Positive for HIV
B. Negative for HIV
C. Indeterminate
D. Further testing needed

Answer 16

Negative for HIV

These results are not indicative of an HIV infection and may be due to a testing error in the first ELISA assay. Known false-positive ELISA reactions occur in autoimmune diseases, syphilis, alcoholism, and lymphoproliferative diseases. A sample is considered positive for HIV if it is repeatedly positive by ELISA or other screening method and positive by a confirmatory method

Question 17

Interpret the following results for HIV infection.
HIV 1,2 ELISA: positive; HIV-1 Western blot:
indeterminate; HIV-1 p24 antigen: negative
A. Positive for antibodies to human
immunodeficiency virus, HIV-1
B. Positive for antibodies to human
immunodeficiency virus, HIV-2
C. Cross reaction; biological false-positive result
D. Additional testing required

Answer 17

Additional testing required

The indeterminate Western blot and negative
p24 antigen assay indicate that HIV-1 infection is unlikely, However, additional testing is required to determine if the patient has antibodies to HIV-2 or if this could be a false-positive ELISA assay.

Question 18

What is the most likely explanation when antibody tests for HIV are negative but a polymerase chain reaction test performed 1 week later is positive?
A. Probably not HIV infection
B. Patient is in the “window phase” before antibody production
C. Tests were performed incorrectly
D. Clinical signs may be misinterpreted

Answer 18

Patient is in the “window phase” before antibody production

In early seroconversion, patients may not be
making enough antibodies to be detected by
antibody tests. The period between infection with HIV and the appearance of detectable antibodies is called the window phase. Although this period has been reduced to a few weeks by sensitive enzyme immunoassays, patients at high risk or displaying clinical conditions associated with HIV disease should be tested again after waiting several more
weeks.

Question 19

What criteria constitute the classification system for HIV infection?
A. CD4-positive T-cell count and clinical
symptoms
B. Clinical symptoms, condition, duration, and
number of positive bands on Western blot
C. Presence or absence of lymphadenopathy
D. Positive bands on Western blot and CD8-positive T-cell count

Answer 19

CD4-positive T-cell count and clinical
symptoms

The classification system for HIV infection is based upon a combination of CD4-positive T-cell count (helper T cells) and various categories of clinical symptoms. Classification is important in determining treatment options and the progression of the disease

Question 20

What is the main difficulty associated with the
development of an HIV vaccine?
A. The virus has been difficult to culture; antigen extraction and concentration are extremely laborious
B. Human trials cannot be performed
C. Different strains of the virus are genetically
diverse
D. Anti-idiotype antibodies cannot be developed

Answer 20

Different strains of the virus are genetically
diverse

Vaccine development has been difficult primarily because of the genetic diversity among different strains of the virus, and new strains are constantly emerging. HIV-1 can be divided into two main subtypes designated M (for main) and O (for outlier). The M group is further divided into 9 subgroups, designated A–J (there is no E subgroup), based upon differences in the nucleotide sequence of the gag gene. Two remaining subtypes are designed N (non M and non O) and P (a subtype related to SIVgor). A vaccine has yet to be developed that is effective for all of the subgroups of HIV-1.

Question 21

Which CD4:CD8 ratio is most likely in a patient
with acquired immunodeficiency syndrome (AIDS)?
A. 2:1
B. 3:1
C. 2:3
D. 1:2

Answer 21

1:2

An inverted CD4:CD8 ratio (less than 1.0) is a common finding in an AIDS patient. The Centers for Disease Control and Prevention requires a CD4-positive (helper T) cell count of less than 200/μL or 14% in the absence of an AIDS-defining illness (e.g., Pneumocystis carinii pneumonia) in the case surveillance definition of AIDS.

Question 22

What is the advantage of 4th-generation rapid
HIV tests over earlier rapid HIV tests?
A. They use recombinant antigens
B. They detect multiple strains of HIV
C. They detect p24 antigen
D. They are quantitative

Answer 22

They detect p24 antigen

Both 3rd-generation and 4th-generation rapid tests for HIV use recombinant and synthetic HIV antigens conjugated to a solid phase. The multivalent nature of these tests allows for detection of less common subgroups of HIV-1 and simultaneous detection of both HIV-1 and HIV-2. However, the 4th-generation assays also use solid-phase antibodies to p24 antigen to detect its presence. Because p24 antigen appears before antibodies to HIV, 4th-generation tests can detect infection 4–7 days earlier than tests based on antibody detection alone.

Question 23

Which method is used to test for HIV infection in infants who are born to HIV-positive mothers?
A. ELISA
B. Western blot test
C. Polymerase chain reaction
D. Viral culture

Answer 23

Polymerase chain reaction

ELISA and Western blot primarily reflect the presence of maternal antibody. The PCR uses small amounts of blood and does not rely on the antibody response. PCR amplifies small amounts of viral nucleic acid and can detect less than 200 copies of viral RNA per milliliter of plasma. These qualities make PCR ideal
for the testing of infants. Nucleic acid methods for HIV RNA include the Roche Amplicor reverse-transcriptase assay, the branched DNA (bDNA) signal amplification method, and the nucleic acid sequence-based amplification (NASBA) method

Question 24

What is the most likely cause when a Western blot or ELISA is positive for all controls and samples?
A. Improper pipetting
B. Improper washing
C. Improper addition of sample
D. Improper reading

Answer 24

Improper washing

Improper washing may not remove unbound
enzyme conjugated anti-human globulin, and every sample may appear positive.

Question 25

What constitutes a diagnosis of viral hepatitis?
A. Abnormal test results for liver enzymes
B. Clinical signs and symptoms
C. Positive results for hepatitis markers
D. All of these options

Answer 25

All of these options

To diagnose a case of hepatitis, the physician must consider clinical signs as well as laboratory tests that measure liver enzymes and hepatitis markers.

Question 26

Which of the following statements regarding
infection with hepatitis D virus is true?
A. Occurs in patients with HIV infection
B. Does not progress to chronic hepatitis
C. Occurs in patients with hepatitis B
D. Is not spread through blood or sexual contact

Answer 26

Occurs in patients with hepatitis B

Hepatitis D virus is an RNA virus that requires the surface antigen or envelope of the hepatitis B virus for entry into the hepatocyte. Consequently, hepatitis D virus can infect only patients who are coinfected with hepatitis B

Question 27

All of the following hepatitis viruses are spread
through blood or blood products except:
A. Hepatitis A
B. Hepatitis B
C. Hepatitis C
D. Hepatitis D

Answer 27

Hepatitis A

Hepatitis A is spread through the fecal–oral route and is the cause of infectious hepatitis. Hepatitis A virus has a shorter incubation period (2–7 weeks) than hepatitis B virus (1–6 months). Epidemics of hepatitis A virus can occur, especially when food and water become contaminated with raw sewage. Hepatitis E
virus is also spread via the oral–fecal route and, like hepatitis A virus, has a short incubation period.

Question 28

Which hepatitis B marker is the best indicator of early acute infection?
A. HBsAg
B. HBeAg
C. Anti-HBc
D. Anti-HBs

Answer 28

HBsAg

Hepatitis B surface antigen (HBsAg) is the first
marker to appear in hepatitis B virus infection. It is usually detected within 4 weeks of exposure (prior to the rise in transaminases) and persists for about 3 months after serum enzyme levels return to normal.

Question 29

Which is the first antibody detected in serum
after infection with hepatitis B virus (HBV)?
A. Anti-HBs
B. Anti-HBc IgM
C. Anti-HBe
D. All are detectable at the same time

Answer 29

Anti-HBc IgM

Antibody to the hepatitis B core antigen (anti-HBc) is the first detectable hepatitis B antibody. It persists in the serum for 1–2 years postinfection and is found in the serum of asymptomatic carriers of HBV. Because levels of total anti-HBc are high after recovery, IgM anti-HBc is a more useful marker for acute infection. Both anti-HBc and anti-HBs can persist for life, but only anti-HBs is considered protective.

Question 30

Which antibody persists in low-level carriers of
hepatitis B virus?
A. IgM anti-HBc
B. IgG anti-HBc
C. IgM anti-HBe
D. IgG anti-HBs

Answer 30

IgG anti-HBc

IgG antibodies to the hepatitis B core antigen
(anti-HBc) can be detected in carriers who are
HBsAg and anti-HBs negative. These persons are presumed infective even though the level of HBsAg is too low to detect. No specific B core IgG test is available, however. This patient would be positive in the anti-B core total antibody assay and negative in the anti-HB core IgM test.

Question 31

What is the most likely explanation when a
patient has clinical signs of viral hepatitis but tests negative for hepatitis A IgM, hepatitis B surface antigen, and hepatitis C Ab?
A. Tests were performed improperly
B. The patient does not have hepatitis
C. The patient may be in the “core window”
D. Clinical evaluation was performed improperly

Answer 31

The patient may be in the “core window”

The patient may be in the “core window,” the period of hepatitis B infection when both the surface antigen and surface antibody are undetectable. The IgM anti-hepatitis B core and the anti-hepatitis B core total antibody assays would be the only detectable markers in the serum of a patient in the core window phase of hepatitis B infection.

Question 32

Which hepatitis B markers should be performed on blood products?
A. HBsAg and anti-HBc
B. Anti-HBs and anti-HBc
C. HBeAg and HBcAg
D. Anti-HBs and HBeAg

Answer 32

HBsAg and anti-HBc

Blood products are tested for HBsAg, an early
indicator of infection, and anti-HBc, a marker that may persist for life. Following recovery from HBV infection, some patients demonstrate negative serology for HBsAg and anti-HBs but are positive for anti-HBc. Such patients are considered infective

Question 33

Which hepatitis antibody confers immunity
against reinfection with hepatitis B virus?
A. Anti-HBc IgM
B. Anti-HBc IgG
C. Anti-HBe
D. Anti-HBs

Answer 33

Anti-HBs

Anti-HBs appears later in infection than anti-HBc and is used as a marker for immunity following infection or vaccination rather than for diagnosis of current infection.

Question 34

Which test, other than serological markers, is most consistently elevated in viral hepatitis?
A. Antinuclear antibodies
B. Alanine aminotransferase (ALT)
C. Absolute lymphocyte count
D. Lactate dehydrogenase

Answer 34

Alanine aminotransferase (ALT)

ALT is a liver enzyme and may be increased in hepatic disease. Highest levels occur in acute viral hepatitis, reaching 20–50 times the upper limit of normal

Question 35

If only anti-HBs is positive, which of the following can be ruled out?
A. Hepatitis B virus vaccination
B. Distant past infection with hepatitis B virus
C. Hepatitis B immune globulin (HBIG) injection
D. Chronic hepatitis B virus infection

Answer 35

Chronic hepatitis B virus infection

Persons with chronic HBV infection show a positive test result for anti-HBc (IgG or total) and HBsAg but not anti-HBs. Patients with active chronic hepatitis have not become immune to the virus.

Question 36

Interpret the following results for EBV infection: IgG and IgM antibodies to viral capsid antigen (VCA) are positive.
A. Infection in the past
B. Infection with a mutual enhancer virus such
as HIV
C. Current infection
D. Impossible to interpret; need more information

Answer 36

Current infection

Antibodies to both IgG and IgM VCA are found in a current infection with EBV. The IgG antibody may persist for life, but the IgM anti-VCA disappears within 4 months after the infection resolves.

Question 37

Which statement concerning non-Forssman
heterophile antibody is true?
A. It is not absorbed by guinea pig antigen
B. It is absorbed by guinea pig antigen
C. It does not agglutinate horse RBCs
D. It does not agglutinate sheep RBCs

Answer 37

It is not absorbed by guinea pig antigen

Non-Forssman antibody is not absorbed by guinea pig antigen. This is one of the principles of the Davidsohn differential test for antibodies to IM. These antibodies are non-Forssman; they are absorbed by sheep, horse, or beef RBCs but not by guinea pig kidney. Therefore, a heterophile titer remaining higher after absorption with guinea pig kidney than
with beef RBCs indicates IM.

Question 38

Blood products are tested for which virus before being transfused to newborns?
A. EBV
B. Human T-lymphotropic virus II (HTLV-II)
C. Cytomegalovirus (CMV)
D. Hepatitis D virus

Answer 38

Cytomegalovirus (CMV)

CMV can be life threatening if transmitted to a
newborn through a blood product. HTLV-II is a rare virus, which like HIV, is a T-cell tropic RNA retrovirus. The virus has been associated with hairy cell leukemia, but this is not a consistent finding

Question 39

What is the endpoint for the antistreptolysin O
(ASO) latex agglutination assay?
A. Highest serum dilution that shows no
agglutination
B. Highest serum dilution that shows agglutination
C. Lowest serum dilution that shows agglutination
D. Lowest serum dilution that shows no
agglutination

Answer 39

Highest serum dilution that shows agglutination

The latex test for ASO includes latex particles coated with streptolysin O. Serial dilutions are prepared and the highest dilution showing agglutination is the endpoint.

Question 40

Interpret the following ASO results:
Tube Nos. 1–4 (Todd unit 125): no hemolysis; Tube No. 5 (Todd unit 166): hemolysis
A. Positive Todd unit 125
B. Positive Todd unit 166
C. No antistreptolysin O present
D. Impossible to interpret

Answer 40

Positive Todd unit 125

An ASO titer is expressed in Todd units as the last tube that neutralizes (no visible hemolysis) the streptolysin O (SLO). Most laboratories consider an ASO titer significant if it is 166 Todd units or higher. However, people with a recent history of streptococcal infection may demonstrate an ASO titer of 166 or higher;
demonstration of a rise in titer from acute to
convalescent serum is required to confirm a current streptococcal infection. ASO is commonly measured using a rapid latex agglutination assay. These tests show agglutination when the ASO concentration is
200 IU/mL or higher.

Question 41

Which control shows the correct result for a valid ASO test?
A. SLO control, no hemolysis
B. Red cell control, no hemolysis
C. Positive control, hemolysis in all tubes
D. Hemolysis in both SLO and red cell control

Answer 41

Red cell control, no hemolysis

The red cell control contains no SLO and should show no hemolysis. The SLO control contains no serum and should show complete hemolysis. An ASO titer cannot be determined unless both the RBC and SLO controls demonstrate the expected results.

Question 42

A streptozyme test was performed, but the result was negative, even though the patient showed clinical signs of a streptococcal throat infection. What should be done next?
A. Either ASO or anti-deoxyribonuclease B
(anti-DNase B) testing
B. Another streptozyme test using diluted serum
C. Antihyaluronidase testing
D. Wait for 3–5 days and repeat the streptozyme test

Answer 42

Either ASO or anti-deoxyribonuclease B
(anti-DNase B) testing

A streptozyme test is used for screening and
contains several of the antigens associated with streptococcal products. Because some patients produce an antibody response to a limited number of streptococcal products, no single test is sufficiently sensitive to rule out infection. Clinical sensitivity is increased by performing additional tests when initial results are negative. The streptozyme test generally shows more false positives and false negatives than ASO and anti-DNase. A positive test
for antihyaluronidase occurs in a smaller number of patients with recent streptococcal infections than ASO and anti-DNase.

Question 43

Rapid assays for influenza that utilize specimens obtained from nasopharyngeal swabs detect:
A. IgM anti-influenza
B. IgA anti-influenza
C. IgA-influenza Ag immune complexes
D. Influenza antigen

Answer 43

Influenza antigen

The rapid influenza assays are antigen detection methods. They are designed to detect early infection, before antibody is produced.

Question 44

How can interfering cold agglutinins be removed from a test sample?
A. Centrifuge the serum and remove the top layer
B. Incubate the clot at 1°C–4°C for several hours, then remove serum
C. Incubate the serum at 56°C in a water bath for 30 minutes
D. Use an anticoagulated sample

Answer 44

Incubate the clot at 1°C–4°C for several hours, then remove serum

Cold agglutinins will attach to autologous red cells if incubated at 1°C–4°C. The absorbed serum will be free of cold agglutinins

Question 45

All tubes (dilutions) except the negative control are positive for cold agglutinins. This indicates:
A. Contaminated red cells
B. A rare antibody against red cell antigens
C. The sample was stored at 4°C prior to separating serum and cells
D. Further serial dilution is necessary

Answer 45

Further serial dilution is necessary

Cold agglutinins may be measured in patients who have cold agglutinin disease, a cold autoimmune hemolytic anemia. In such cases, titers can be as high as 106. If all tubes (dilutions) for cold agglutinins are positive, except the negative control, then a high titer of cold agglutinins is present in the sample. Further serial dilutions should be performed.

Question 46

All positive cold agglutinin tubes remain positive after 37°C incubation except the positive control. What is the most likely explanation for this situation?
A. High titer cold agglutinins
B. Contamination of the test system
C. Antibody other than cold agglutinins
D. Faulty water bath

Answer 46

Antibody other than cold agglutinins

Cold agglutinins do not remain reactive above 30°C, and agglutination must disperse following incubation at 37°C. The most likely explanation when agglutination remains after 37°C incubation is that a warm alloantibody or autoantibody is present.

Question 47

Which increase in antibody titer (dilution) best
indicates an acute infection?
A. From 1:2 to 1:8
B. From 1:4 to 1:16
C. From 1:16 to 1:256
D. From 1:64 to 1:128

Answer 47

From 1:16 to 1:256

A fourfold (2 tube) or greater increase in antibody titer is usually indicative of an acute infection. Although answers A and B show a fourfold rise in titer, answer C shows a 16-fold rise in titer and is the most definitive. In most serological tests, a single high titer is insufficient evidence of acute infection unless
specific IgM antibodies are measured because age, individual variation, immunologic status, and history of previous exposure (or vaccination) cause a wide variation in normal serum antibody titers.

Question 48

Which of the following positive antibody tests
may be an indication of recent vaccination or early primary infection for rubella in a patient with no clinical symptoms?
A. Only IgG antibodies positive
B. Only IgM antibodies positive
C. Both IgG and IgM antibodies positive
D. Fourfold rise in titer for IgG antibodies

Answer 48

Only IgM antibodies positive

If only IgM antibodies are positive, this result indicates a recent vaccination or an early primary infection.

Question 49

Why is laboratory diagnosis difficult in cases of
Lyme disease?
A. Clinical response may not be apparent upon
initial infection; IgM antibody may not be
detected until 3–6 weeks after the infection
B. Laboratory tests may be designed to detect whole Borrelia burgdorferi, not flagellar antigen found early in infection
C. Most laboratory tests are technically demanding and lack specificity
D. Antibodies formed initially to B. burgdorferi may cross react in antigen tests for autoimmune diseases

Answer 49

Clinical response may not be apparent upon
initial infection; IgM antibody may not be
detected until 3–6 weeks after the infection

Lyme disease is caused by B. burgdorferi, a spirochete, and typical clinical symptoms such as rash or erythema chronicum migrans may be lacking in some infected individuals. Additionally, IgM antibody is not detectable by laboratory tests until 3–6 weeks after a tick bite, and IgG antibody develops later.

Question 50

Serological tests for which disease may give a falsepositive result if the patient has Lyme disease?
A. AIDS
B. Syphilis
C. Cold agglutinins
D. Hepatitis C

Answer 50

Syphilis

Lyme disease is caused by a spirochete and may give positive results with some specific treponemal antibody tests for syphilis.

Question 51

In monitoring an HIV-infected patient, which
parameter may be expected to be the most
sensitive indicator of the effectiveness of
antiretroviral treatment?
A. HIV antibody titer
B. CD4:CD8 ratio
C. HIV viral load
D. Absolute total T-cell count

Answer 51

HIV viral load

The HIV viral load will rise or fall in response to
treatment more quickly than any of the other listed parameters. The absolute CD4 count is also an indicator of treatment effectiveness and is used in resource-poor areas that might not have facilities for molecular testing available. Note that the absolute CD4 count is not one of the choices, however.

Question 52

A renal transplant recipient is found to have a
rising creatinine level and reduced urine output. The physician orders a “Urine PCR” assay. When you call to find out what organism the physician wants to identify, you are told:
A. Hepatitis C virus
B. Legionella pneumophila
C. EBV
D. BK virus

Answer 52

BK virus

BK virus is a polyoma virus that can cause renal and urinary tract infections. The virus is an opportunistic pathogen and has become a well-recognized cause of poor renal function in kidney transplant recipients. Antibody testing is not practical or useful for this infection. The principal diagnostic assays are urinary cytology, and specific BK virus PCR testing in urine and serum. Although Legionella pneumophila can be diagnosed through a urinary antigen assay, that organism is not a primary cause of renal insufficiency in transplant patients.

Question 53

A newborn is to be tested for a vertically
transmitted HIV infection. Which of the
following tests is most useful?
A. HIV PCR
B. CD4 count
C. Rapid HIV antibody test
D. HIV IgM antibody test

Answer 53

HIV PCR

Neonatal HIV diagnosis is performed by screening for the presence of the virus. The current antibody tests are either IgG-specific or an IgG/IgM combination assay. Thus an infant whose mother is HIV positive will also be positive in the HIV antibody assay. Although the CD4 count may be a useful assay to
determine disease activity, there are many causes of reduced CD4 numbers and this assay should not be used to diagnose HIV infection

Question 54

Which of the following methods used for HIV
identification is considered a signal amplification technique?
A. Branched chain DNA analysis
B. DNA PCR
C. Reverse transcriptase PCR
D. Nucleic acid sequence based assay (NASBA)

Answer 54

Branched chain DNA analysis

Branched chain DNA is a signal amplification
technique, i.e., if you start with one copy of the gene you finish with one copy. The detection reagent is amplified, increasing the sensitivity of the assay.

Question 55

Which of the following fungal organisms is best
diagnosed by an antibody detection test as opposed to an antibody detection assay?
A. Histoplasma
B. Cryptococcus
C. Candida
D. Aspergillus

Answer 55

Cryptococcus

The Cryptococcus antibody response is not a reliable indicator of a current infection; thus, an antigen assay is normally used to monitor the disease. The antigen assay may be used for serum or spinal fluid and will decline in response to treatment much faster than a traditional antibody test. A urinary antigen test is avail - able for histoplasmosis, and a serum galactomannan assay is available for Aspergillus. Those two assays preform better than antibody detection. No antigen test is available for Candida, thus antibody is the best
serologic procedure for this organism.

Question 56

Your cytology laboratory refers a Papanicolaou
smear specimen to you for an assay designed to detect the presence of a virus associated with cervical cancer. You perform:
A. An ELISA assay for anti-HSV-2 antibodies
B. A molecular assay for HSV-2
C. An ELISA assay for HPV antibodies
D. A molecular assay for HPV

Answer 56

A molecular assay for HPV

Cervical cell atypia and cervical cancer are associated with specific high-risk serotypes of human papilloma virus (HPV) infections. Although HPV antibody assays are available, they are not serotype specific, nor do they relate to disease activity. Thus molecular probe
assays are the tests of choice to detect high-risk HPV infection. Although HSV-2 is associated with genital herpesvirus, that virus has not been shown to cause cervical cancer

Question 57

An immunosuppressed patient has an unexplained anemia. The physician suspects a parvovirus B19 infection. A parvovirus IgM test is negative. The next course of action is to tell the physician:
A. The patient does not have parvovirus
B. A convalescent specimen is recommended in 4 weeks to determine if a fourfold rise in titer has occurred
C. A parvovirus PCR is recommended
D. That a recent transfusion for the patient’s anemia may have resulted in a false-negative assay and the patient should be retested in 4 weeks

Answer 57

A parvovirus PCR is recommended

A negative IgM assay rarely rules out an infection. While a convalescent specimen may be useful in many cases, in an immunosuppressed patient the convalescent specimen may remain negative in the presence of an infection. Thus a parvovirus PCR test is
the preferred choice in this case. A false-negative result could conceivably be caused by multiple whole blood or plasma transfusions, but retesting for antibody a month later would not be beneficial to the patient.