MLPAO Clinical Chemistry Advanced Concepts 2 — Questions and Answers
Question 1: What is the primary advantage of using enzymatic creatinine assays over the Jaffe method?
- Lower cost
- Greater specificity with fewer interferences (Correct answer)
- Faster reaction time
- Wider analytical range
Correct answer: Greater specificity with fewer interferences
Enzymatic creatinine assays offer greater specificity than the Jaffe method because they are less susceptible to interference from non-creatinine chromogens such as glucose, acetoacetate, cephalosporins, and bilirubin. This is particularly important in neonatal and pediatric samples.
Question 2: Which calculation is used to estimate glomerular filtration rate (eGFR) in Canadian clinical practice?
- Cockcroft-Gault equation only
- CKD-EPI equation (Correct answer)
- MDRD equation only
- Creatinine clearance from 24-hour urine
Correct answer: CKD-EPI equation
The CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation is recommended for estimating GFR in Canadian clinical practice. It uses serum creatinine, age, and sex and is more accurate than the older MDRD equation, particularly at higher GFR values.
Question 3: What causes a delta check failure in the clinical chemistry laboratory?
- A QC result exceeding 2 standard deviations
- A significant difference between a patient's current and previous result (Correct answer)
- An instrument calibration failure
- A reagent lot change
Correct answer: A significant difference between a patient's current and previous result
A delta check compares a patient's current test result with their previous result and flags significant changes that may indicate specimen misidentification, sample mix-up, or acute clinical change. The acceptable delta varies by analyte and is typically expressed as a percentage or absolute change.
Question 4: Which hormone is measured using a stimulation test with cosyntropin (synthetic ACTH)?
- Cortisol (Correct answer)
- Aldosterone
- Testosterone
- Growth hormone
Correct answer: Cortisol
The cosyntropin stimulation test measures cortisol response to synthetic ACTH to assess adrenal gland function. A normal response shows cortisol rising above 500 nmol/L, while an inadequate response suggests primary adrenal insufficiency (Addison disease).
Question 5: What is the principle of the ion-selective electrode used for measuring ionized calcium?
- Amperometry using a platinum electrode
- Potentiometry using a calcium-selective membrane (Correct answer)
- Conductimetry measuring total ionic strength
- Coulometry using an electrolytic cell
Correct answer: Potentiometry using a calcium-selective membrane
Ionized calcium is measured by potentiometry using an electrode with a calcium-selective membrane (typically containing a calcium-selective ionophore). The electrode generates a voltage proportional to the logarithm of ionized calcium activity in the specimen.
Question 6: Why is it important to measure ionized calcium rather than total calcium in critically ill patients?
- Ionized calcium is more stable in storage
- Ionized calcium represents the biologically active form unaffected by albumin changes (Correct answer)
- Total calcium cannot be measured in ICU settings
- Ionized calcium is less expensive to measure
Correct answer: Ionized calcium represents the biologically active form unaffected by albumin changes
Ionized calcium is the physiologically active form and is preferred in critically ill patients because total calcium is significantly affected by albumin levels, which are frequently abnormal in these patients. Approximately 40% of total calcium is bound to albumin, so hypoalbuminemia can mask true calcium status.
MLPAO Clinical Chemistry Advanced Concepts 2
What is the primary advantage of using enzymatic creatinine assays over the Jaffe method?