MLPAO Clinical Chemistry Laboratory Practice 1 — Questions and Answers
Question 1: What is the purpose of a reagent blank in spectrophotometric clinical chemistry assays?
- To verify patient results are accurate
- To correct for absorbance contributed by reagents and cuvette rather than the analyte (Correct answer)
- To calibrate the wavelength of the spectrophotometer
- To measure the concentration of the calibrator
Correct answer: To correct for absorbance contributed by reagents and cuvette rather than the analyte
A reagent blank measures the absorbance of all components except the analyte of interest (reagents, diluent, cuvette). This value is subtracted from the sample reading to ensure that only the absorbance due to the analyte is measured, improving accuracy.
Question 2: Which analyte is most affected by improper specimen transport temperature?
- Sodium
- Ammonia (Correct answer)
- Total protein
- Chloride
Correct answer: Ammonia
Ammonia is extremely sensitive to transport conditions. Specimens must be placed on ice immediately after collection and analyzed within 15-30 minutes. At room temperature, deamination of amino acids and other nitrogen-containing compounds causes rapid ammonia generation, leading to falsely elevated results.
Question 3: What is the clinical significance of measuring cystatin C?
- Assessing liver fibrosis
- Estimating GFR independent of muscle mass (Correct answer)
- Diagnosing pancreatitis
- Monitoring warfarin therapy
Correct answer: Estimating GFR independent of muscle mass
Cystatin C is a small protein produced at a constant rate by all nucleated cells and freely filtered by the glomerulus. Unlike creatinine, cystatin C is not significantly affected by muscle mass, age, or sex, making it a more reliable GFR marker in patients with extremes of body composition.
Question 4: Which Westgard rule is considered a 'warning' rule rather than a rejection rule?
- 1-3s
- 1-2s (Correct answer)
- 2-2s
- R-4s
Correct answer: 1-2s
The 1-2s rule is a warning rule that triggers inspection of additional QC data. A single QC result exceeding 2 standard deviations from the mean occurs randomly about 5% of the time and does not alone indicate an analytical problem. It prompts checking other Westgard rules for true violations.
Question 5: What is Beer's Law and how does it apply to clinical chemistry?
- It describes the relationship between temperature and reaction rate
- It states that absorbance is directly proportional to the concentration of the absorbing substance and path length (Correct answer)
- It defines the relationship between pH and pKa
- It describes the equilibrium between bound and free drug
Correct answer: It states that absorbance is directly proportional to the concentration of the absorbing substance and path length
Beer's Law (A = εbc) states that absorbance is directly proportional to the molar absorptivity (ε), path length (b), and concentration (c) of the absorbing substance. This is the fundamental principle underlying spectrophotometric measurements in clinical chemistry, allowing concentration to be determined from absorbance readings.
Question 6: Which preanalytical factor causes pseudohyperkalemia?
- Patient dehydration
- Fist clenching during venipuncture (Correct answer)
- Use of a serum separator tube
- Short tourniquet application
Correct answer: Fist clenching during venipuncture
Repeated fist clenching during venipuncture causes potassium release from forearm muscle cells, leading to pseudohyperkalemia. Patients should be instructed to keep their fist still during blood collection. Other causes include hemolysis and extreme leukocytosis or thrombocytosis.
MLPAO Clinical Chemistry Laboratory Practice 1
What is the purpose of a reagent blank in spectrophotometric clinical chemistry assays?