1. (M1) A 71-year-old man has had pain in his mouth for the past 10 days. On examination there is pharyngeal swelling, erythema, and exudate. Which of the following preanalytical variables is most likely to complicate identification of the causative organisms?
A. Low concentration of organisms
B. Presence of normal microflora
C. Sample collection technique
D. Specimen transport
E. Timing of sample collection
Answer: B: Presence of normal microflora.
The oral and pharyngeal regions have an abundant normal microflora. Since the site can be visualized and accessed directly, obtaining the specimen should not be a significant issue.
2. (M3) A respiratory culture is obtained, but an unusual organism, Rhodococcus equi, is suspected as the causative organism. Which of the following components of laboratory testing is most likely to impact the receipt of results by the ordering physician?
A. Preanalytical
B. Analytical
C. Postanalytical
Answer: B: Analytical.
A fastidious organism will take time to grow in culture. Even if immunological or other testing such as PCR is performed, it is likely that the test is not run every day. The test is likely to be performed in a reference lab, but shipping the specimen is likely to take no more than a day. Once a result is obtained, it can be transmitted electronically.
3. (M4) A 33-year-old woman has had low-grade fever with night sweats and productive cough for the past month. Her chest x-ray shows upper lobe granulomatous disease. A positive result for which of the following is the best evidence for use of immediate antimicrobial therapy and patient isolation?
A. C-reactive protein
B. HIV test
C. Interferon gamma release assay
D. Sputum with acid fast bacilli
E. Tuberculin skin test
Answer: D: Sputum with acid fast bacilli.
The presence of acid fast bacilli indicates not only active infection but also a hazard to others.
4. (M5) A 24-year-old health care worker incurs a needlestick injury while performing phlebotomy. A blood sample is drawn on this worker within a day following the injury, and the following test results are obtained on that sample:
Test
Result
Hepatitis A, IgM
negative
Hepatitis A, IgG
positive
Hepatitis B, core antibody
negative
Hepatitis B, surface antigen
positive
Hepatitis B, surface antibody
negative
Hepatitis C antigen
negative
What is the most likely interpretation of these findings?
A. Hepatitis A acute infection
B. Hepatitis B, carrier status
C. Hepatitis B, needlestick exposure
D. Hepatitis B, prior vaccination
E. Hepatitis C, chronic infection
Answer: B: Hepatitis B, carrier status.
The hepatitis A IgG represents past exposure, which many people have, but most of these are trivial, subclinical infections. Many health care workers have received hepatitis B vaccine, and the measure of the success of this vaccination is the presence of hepatitis B surface antibody. Hence, this health care worker should have protected against infection with hepatitis B, but either did not receive the vaccine, or failed to seroconvert following vaccination. An immune response with detectable antibody generally takes more than a week to occur. The presence of hepatitis B surface antigen without antibody suggests chronic infection, and if the surface antigen persists longer than 6 months then a carrier status is present, though most of these persons will eventually clear the virus, and a subset remain 'healthy' carriers. Though this health care worker does not have evidence at this time for hepatitis C infection, the incubation period of a month or more means that exposure could have occurred from this needlestick.
5. (M6) A 33-year-old man has acute infective endocarditis with blood culture positive for Streptococcus, viridans group. He is given an antibiotic regimen that includes gentamicin. A test is performed for the minimum inhibitory dilution (MID) for gentamicin based upon patient serum dilutions for both peak and trough antibiotic concentrations. The findings are shown (0 at the right is the control, with no antibiotic):
Trough
Peak
What is the MID that best predicts good antibiotic effectiveness?
A. 1:2
B. 1:4
C. 1:8
D. 1:16
E. 1:32
F. 1:64
G. 1:128
Answer: E: 1:32.
One may want to know what effectiveness an antibiotic has in the patient serum. A minimum inhibitory dilution (MID) may be performed by taking a patient serum sample, making serial dilutions, and placing those patient serum dilutions into broth growing the organism that has been isolated from the patient. The MID is performed to correlate with doses of the antibiotic given and measured in the patient serum as peak and trough levels just after and just before administration of the dose, respectively. The MID is at a 1:4 dilution for the trough level of gentamicin, and at 1:32 for the peak level. In general, an MID of 1:32 suggests good antibiotic effectiveness.
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