Rapid Tests (Basic Info, Limitations)

Question 1

Cepheid Xpert C. difficile - What does it detect?

Answer 1

Toxin B gene sequences

Question 2

Cepheid Xpert C. difficile - What is appropriate sample?

Answer 2

Unformed stool

Question 3

Cepheid Xpert C. difficile - How many toxins does C. diff have? What are their names?

Answer 3

Toxin A (tcdA) - enterotoxin
Toxin B (tcdB) - (req for pathogenic sp)
CDT (cdtA, cdtB) - binary toxin

Question 4

Cepheid Xpert C. difficile - How often is this test preformed per patient?

Answer 4

Once every 7 days

Question 5

Cepheid Xpert C. difficile - What are the internal quality controls?

Answer 5

Sample Processing Control (SPC) - Bacillus globigii spores, verifies adequate lysis, detects specimen inhibition of PCR

Probe Check Control (PCC) - before rxn starts measures fluorescence of probes to check probe integrity, rehydration, dye stability

Question 6

Cepheid Xpert C. difficile - Limitations?

Answer 6

• False negs reported in presence of mucin
• Does not differentiate toxigenic C. diff strains
• Will only detect strains with tcdB
• Inhibition of assay reported for zinc oxide paste, Vagisil cream
• Due to dilution, retests of low pos may be neg

Question 7

Cepheid Xpert MRSA - Primary Use?

Answer 7

Surveillance

Question 8

Cepheid Xpert MRSA - What gene encodes methicillin-resistance?

Answer 8

mecA

Question 9

Cepheid Xpert MRSA - What does it detect?

Answer 9

A proprietary sequence of a cassette inserted into the S. aureus chromosome

Question 10

Cepheid Xpert MRSA - What is appropriate sample?

Answer 10

Nasal swabs (2/collection)

Question 11

Cepheid Xpert MRSA - What are the internal quality controls?

Answer 11

Sample Processing Control (SPC) - Bacillus globigii spores, verifies adequate lysis, detects specimen inhibition of PCR

Probe Check Control (PCC) - before rxn starts measures fluorescence of probes to check probe integrity, rehydration, dye stability

Question 12

Cepheid Xpert MRSA - What to do if positive result?

Answer 12

Notify physician and epidemiology

Inoculate second swab into MRSA broth (MH broth w/ NaCl and Oxacil), leave result as preliminary until Bacti confirms by culture

Question 13

Cepheid Xpert MRSA - Limitations?

Answer 13

Positive result does not guarantee presence of viable organism

Question 14

How many van genes are there?

Answer 14

6 - vanA, vanB, vanC, vanD, vanE, and vanG

Question 15

Which van genes are most clinically important?

Answer 15

vanA (high resistance)

vanB (moderate resistance)

Question 16

What other orgs carry van genes?

Answer 16

vanA - Staph aureus (9 reported in US), Strep (rare)

vanB - more disseminated among intestinal anaerobes

Question 17

Cepheid Xpert vanA - What is appropriate sample?

Answer 17

Perirectal swab (2/collection)

Question 18

Cepheid Xpert vanA - What are the internal quality controls?

Answer 18

Sample Processing Control (SPC) - Bacillus globigii spores, verifies adequate lysis, detects specimen inhibition of PCR

Probe Check Control (PCC) - before rxn starts measures fluorescence of probes to check probe integrity, rehydration, dye stability

Question 19

Cepheid Xpert vanA - What to do if positive result?

Answer 19

Notify physician and epidemiology

Inoculate second swab into VRE broth (BEA broth w/ vanc), leave result as preliminary until Bacti confirms by culture

Question 20

Cepheid Xpert vanA - Limitations?

Answer 20

• ability to amplify vanA from non-entercoccal species unknown
• hydrocortisone and pepto-bismol may interfere with assay
• detects only vanA, not the org that carries it (may be Staph)
• positive result does not guarantee presence of viable organism
• mutations is primer sites may effect detection of vanA variants

Question 21

Occult Blood - How does the rxn work?

Answer 21

Heme (from hemoglobin) catalyses the oxidation of guaiac (test paper) by hydrogen peroxide into a blue quinone compound

Question 22

Occult Blood - Limitations?

Answer 22

• blood may not be uniform in stool
• not suitable for gastric specimens
• blood loss thru stool can occur in healthy individuals