interpretation of clinical lab tests

Question 1

if a test is highly sensitive, it means?

Answer 1

• test result is negative you can be nearly certain that the patient does not have the disease
• high rate of true negatives

Question 2

highly sensitive tests are used to ______ for disease

Answer 2

screen

Question 3

if a test is highly specific, it means?

Answer 3

• test result is positive you can be nearly certain that patient actually has disease
• high rate of true positives

Question 4

highly specific tests are used to _____ for disease

Answer 4

confirm the results of sensitive

Question 5

validity

Answer 5

measure of the test’s ability to indicate which individuals have the disease and which do not

Question 6

reliability

Answer 6

• another term for consistency

- if test is admin repeatedly, does it yield the same results

Question 7

incidence

Answer 7

• the number of new cases of a disease or condition in a specified time period divided by the size of the population under consideration who were initially disease free

Question 8

incidence is a direct measure of?

Answer 8

disease risk - high incidence implies higher risk

Question 9

prevalence

Answer 9

• actual number of cases alive with the disease either during a period of time (period prevalence) or at a particular date in time (point prevalence)

Question 10

period prevalence vs point prevalence

Answer 10

• period prevalence provides better measure of the disease load since it includes all new cases and all deaths between 2 dates
• point prevalence only counts those alive on a particular date

Question 11

low prevalence rate means

Answer 11

a lot of ppl die from the disease or the disease is cured fairly quickly

Question 12

high prevalence rate means

Answer 12

a lot of ppl live with the disease

Question 13

screening test is used to?

Answer 13

identify an asymptomatic individual who may have a particular disease

Question 14

diagnostic test is sued to?

Answer 14

confirm the presence of a disease when a subject shows signs or symptoms of the disease

Question 15

common screening tests

Answer 15

1. pap smear
2. fasting blood cholesterol
3. fasting blood sugar
4. BP
5. fecal occult blood
6. ocular pressure
7. PKU test
8. TSH

Question 16

lab tests to workup for chest pain

Answer 16

1. serum lactate dehydrogenase level
2. serum creatinine phosphokinase level
3. serum myoglobin levels
4. serum troponin levels

Question 17

lactate dehydrogenase

Answer 17

• an enzyme that catalyzes the conversion of lactate to pyruvate
• as cells die, their LDH is released and finds its way into the blood

Question 18

lactate dehydrogenase levels in MI

Answer 18

• total LDH level rises within 24-48 hrs after an MI, peaks in 2-3 days, and returns to normal in 5-10 days

Question 19

lactate dehydrogenase sensitivity and specificity for MI

Answer 19

diagnostic sensitivity and specificity is more than 90% within 24 hours of an MI when LDH-1 >40% of total; however even slight hemolysis can cause non-diagnostic elevations in LDH-1

Question 20

creatinine phosphokinase

Answer 20

CPK enters blood rapidly following damage to mm cells but CPK levels rise and fall rapidly with a variety of other circumstances

Question 21

sensitivity and specificity of CPK-MB for MI

Answer 21

• sensitivity and specificity of CPK-MB for MI are >90% within 7-18 hrs; peak concentrations occur within 24 hours and return to normal within 2-3 days
• sensitivity is poor when total CPK is very high
• specificity is poor when total CPK is low

Question 22

myoglobin sensitivity for MI

Answer 22

not sensitivity enough to use for negative predictive value

Question 23

troponin in dx MI

Answer 23

• criterion standard for dx MI
• higher specificity for MI than CK-MB
• higher long-term sensitivity than LD-1

Question 24

troponin subunits seen in MI

Answer 24

cTnI and cTnT (troponins I and T)

Question 25

lab test is useful in identifying the etiology of _____

Answer 25

anemia

Question 26

CBC documents the ___ of the anemia

Answer 26

severity

Question 27

what is shown on a CBC in chronic iron deficiency anemia?

Answer 27

mean corpuscular volume (MCV) and mean corpuscular hemoglobin concentration (MCHC) have values below normal range

Question 28

when will CBC enter abnormal range during blood loss?

Answer 28

CBC will not enter abnormal range until most of the RBC produced before the bleed are destroyed at the end of their normal lifespan (120 days)

Question 29

what will be seen on a peripheral smear in chronic iron deficiency anemia?

Answer 29

- microcytic and hypochromic RBC | - microcytosis is apparent in the smear before the MCV is decreased

Question 30

how can MCV normalize?

Answer 30

- macrocytes mixed among the microcytic hypochromic cells | - occur in areas with folate and iron deficiency due to little fresh produce and meat

Question 31

what is diagnostic of iron deficiency?

Answer 31

low serum iron and ferritin levels with an elevated TIBC

Question 32

more sensitive laboratory indicators of mild iron deficiency

Answer 32

serum ferritin and stainable irone in tissue stores

Question 33

how long before RBC indices become abnormal after tissue stores are depleted of iron?

Answer 33

several months

Question 34

how is stool testing used in dx iron deficiency anemia?

Answer 34

hemoglobin in the stool is useful in establishing GI bleeding as etiology of iron deficiency anemia

Question 35

how can bone marrow aspirate be diagnostic of iron deficiency?

Answer 35

absence of stainable iron in bone marrow aspirate permits establishment of a dx of iron deficiency w/o other lab tests

Question 36

definition of oliguria

Answer 36

diminished capacity to form and pass urine -

Question 37

UA findings in prerenal failure

Answer 37

- few hyaline and fine, granular casts - little protein, heme, or red cells - specific gravity of urine increased

Question 38

UA findings in intrinsic renal failure

Answer 38

- hematuria and proteinuria are prominent - broad, brown, granular casts found in ischemic or toxic acute tubular necrosis - red cell casts found in glomerulonephritis - white cells, especially eosinophils and white cell casts in acute interstitial nephritis

Question 39

what is often used to distinguish decreased GFR from intrinsic renal disease?

Answer 39

fractional excretion of sodium

Question 40

BUN/Cr in pre-renal and renal failure

Answer 40

pre-renal: >20/1 | renal:

Question 41

why is serum creatinine unreliable as indicator of renal damage?

Answer 41

- serum creatinine vary with age, lean mm mass, and hydration status - serum levels may not change until >50% of kidney fx has been lost - at lower GFR, increased tubular secretion of creatinine can lead to overestimation of renal fx

Question 42

serum sodium is useful in dx ____?

Answer 42

cause of oliguria since urine output is influenced by serum sodium

Question 43

hyponatremia usually indicates? hypernatremia usually indicates?

Answer 43

- hyponatremia is usually dilutional secondary to fluid retention or admin of hypotonic fluids - hypernatremia is usually secondary to dehydration

Question 44

how is serum potassium related to oliguria?

Answer 44

oliguria -> decreased GFR -> reduced tubular secretion -> metabolic acidosis -> increased serum potassium (decreased arterial pH raises serum potassium)

Question 45

4 legitimate reasons for ordering a lab test

Answer 45

1. dx 2. monitoring 3. screening 4. research

Question 46

approach to ordering lab tests

Answer 46

1. shotgun - ordering a large number of lab tests that may or may not help in dx a particular disease 2. rifle - ordering a specific lab tests based on how helpful it is in identifying a particular disease