CBET Laboratory and In-Vitro Diagnostic Equipment — Questions and Answers
Question 1: A hematology analyzer using the Coulter principle counts and sizes blood cells by measuring changes in what physical property as cells pass through an aperture?
- Electrical impedance (Correct answer)
- Light scattering angle
- Fluorescence emission
- Magnetic field distortion
Correct answer: Electrical impedance
The Coulter principle (electrical impedance method) works by suspending cells in a conductive electrolyte and passing them through a small aperture. Each cell displaces electrolyte, momentarily increasing resistance. The magnitude of the impedance change corresponds to cell volume, while the frequency of pulses equals cell count.
Question 2: A blood gas analyzer's pO₂ electrode consistently reads 10–15% lower than expected across all QC levels. After confirming the sample path is clear, what is the most likely cause?
- Air bubbles trapped in the QC sample syringe
- A fouled or expired Clark electrode membrane (Correct answer)
- Incorrect barometric pressure input
- Contaminated sodium reference electrode
Correct answer: A fouled or expired Clark electrode membrane
The Clark polarographic electrode measures oxygen via a gas-permeable membrane. As the membrane ages or becomes protein-fouled, it restricts O₂ diffusion to the cathode, causing consistently low pO₂ readings across all control levels. Air bubbles typically cause falsely HIGH readings and affect only individual samples, not all QC levels.
Question 3: During PM on a clinical centrifuge, a BMET notices excessive vibration during a spin cycle. What should be checked FIRST?
- Carbon brushes on the drive motor
- Rotor balance and proper symmetrical tube loading (Correct answer)
- Speed control potentiometer calibration
- Timer relay contacts
Correct answer: Rotor balance and proper symmetrical tube loading
Vibration in a centrifuge is most commonly caused by imbalanced loads — mismatched tube weights or asymmetric placement in the rotor. An imbalanced rotor can cause catastrophic rotor failure. Motor brushes cause speed issues, not vibration, and the speed control and timer are not mechanical vibration sources.
Question 4: Which regulatory standard specifically mandates that clinical laboratories run quality control specimens at defined intervals before reporting patient results?
- ISO 9001 General Quality Management
- CLIA '88 (Clinical Laboratory Improvement Amendments) (Correct answer)
- FDA 510(k) premarket notification
- NFPA 99 healthcare facilities standard
Correct answer: CLIA '88 (Clinical Laboratory Improvement Amendments)
CLIA '88 (Clinical Laboratory Improvement Amendments of 1988) sets federal standards for all U.S. clinical laboratory testing, including requirements to run calibrators and QC at minimum defined frequencies before reporting patient results. ISO 9001 is a generic quality standard, 510(k) governs device clearance, and NFPA 99 covers utilities and medical gases.
Question 5: A spectrophotometer used for hemoglobin measurement is checked and found to have a wavelength accuracy error of ±5 nm at 415 nm. Why is this clinically significant?
- 415 nm is the Soret band of hemoglobin, where absorbance changes sharply with wavelength, causing large measurement errors (Correct answer)
- 415 nm is outside the visible spectrum and the detector sensitivity drops off
- Hemoglobin only absorbs in the infrared range, so UV errors compound
- A 5 nm shift causes the blank reading to exceed the sample reading
Correct answer: 415 nm is the Soret band of hemoglobin, where absorbance changes sharply with wavelength, causing large measurement errors
Hemoglobin's Soret absorption band at ~415 nm is extremely narrow and steep. Even a small wavelength shift (±5 nm) moves the measurement off the peak into a region of dramatically different absorbance, producing significant inaccuracy. This is why wavelength accuracy is a critical PM parameter for spectrophotometers used in clinical labs.
Question 6: A point-of-care blood glucose meter consistently reads 18% higher than the central lab reference method for the same patient samples. After confirming patient prep was correct, the MOST likely cause is:
- The patient has polycythemia, falsely elevating capillary glucose
- The test strip lot has an incorrect calibration code or expired strips (Correct answer)
- The meter's Bluetooth sync introduced a data offset
- The central lab method is known to underestimate in ICU patients
Correct answer: The test strip lot has an incorrect calibration code or expired strips
A consistent percentage bias across multiple patients points to a calibration error rather than a patient-specific issue. Incorrect strip calibration codes or expired strips are the most common cause of systematic bias in POC glucose meters. The meter must be coded to match the electrochemical characteristics of each strip lot.
CBET Laboratory and In-Vitro Diagnostic Equipment
A hematology analyzer using the Coulter principle counts and sizes blood cells by measuring changes in what physical property as cells pass through an aperture?