1. (F1) An urban free clinic in a large city offers testing for infection with syphilis. The clinic uses a screening test designed to obtain maximum sensitivity, followed by a second test performed before results are reported. Which of the following outcomes is most likely to characterize this screening test?
A. Increased false positive tests
B. Decreased false positive tests
C. Increased false negative tests
D. Decreased false negative tests
Answer: A - Increased false positives.
In order to try and detect as many true infections as possible, a greater number of false positives will occur. For most screening serologic tests for syphilis, the false positive rate is at least 1%. Hence, a second confirmatory test is required before reporting a result as a true positive. Even if both the sensitivity and specificity of a test is high, the low prevalence for most infections means that most of the positive screening tests will be false positives. Hence, a specific confirmatory test is needed to detect true positives.
2. (F2) A 50-year-old healthy man has a serum alanine aminotransferase (ALT) test. The value is 21 U/L and the normal range is 8 - 20 U/L. What is the probability that his result falls outside of the normal range?
A. 1 in 20
B. 1 in 10
C. 1 in 4
D. 1 in 2
Answer: A - 1 in 20.
The 95% confidence limits chosen mean that, on average 1 in 20 tests falls outside the normal range.
3. (F6) A 62-year-old woman with atrial fibrillation is taking warfarin. On her latest visit, she reports no problems. A blood specimen is obtained for prothrombin time. How should this test be ordered?
A. Routine turnaround time
B. Stat turnaround time
C. Report abnormal value to the health department
D. Report critical value electronically
E. Draw via phlebotomy into a red top tube
F. Draw via phelbotomy into a purple top tube
Answer: A - Routine turnaround time.
This is a routine test.
4. (F7) An electrolyte panel is obtained for an adult patient with gastroenteritis and diarrhea. Results show sodium 118 mmol/L, potassium 3.1 mmol/L, chloride 88 mmol/L, and HCO3 23 mmol/L. A serum osmolality is then obtained and shows 290 mOsm/kg. Which of the following is the most likely explanation for these findings?
A. Analytical error with instrumentation in the laboratory
B. Concurrent diabetes insipidus with dehydration
C. Fluid overload from intravenous administration of saline
D. Hemolysis in obtaining specimens during phlebotomy
E. Lipemic interference with electrolyte measurement
Answer: E - Lipemic interference with electrolyte measurement.
Lipemia may interfere with analysis of a serum specimen. Lipemia may produce a false dilution. Reduction in electrolyte levels in association with a normal serum osmolality may be the result of lipemia. Most laboratory electrolyte analyzers use an indirect ion selective electrode (ISE) susceptible to volume displacement by lipids. Normal plasma usually contains around 7% protein-lipid and 93% water by volume. If the concentration of triglyceride in plasma is markedly elevated, then the volume of water is significantly reduced but the volume of solution remains unchanged. As a result the molarity (in mmol/L) of ions such as sodium measured by indirect ion selective electrodes (ISE) will be reduced but the molality will be unaltered.
5. (F10) A series of point-of-care glucose measurements from a hospital ward yields glucose values that are all much lower than the corresponding values from specimens drawn and sent to the hospital laboratory. In which of the following phases of testing is the causative problem most likely to be?
A. Preanalytical
B. Analytical
C. Postanalytical
Answer: B - Analytical.
With POC testing, the most common causes for errors involve performing the test.
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