Medical Laboratory Instrumentation
Medical laboratory instrumentation tests how analysers generate a number and how that number can go wrong. Expect spectrophotometry, ion-selective electrodes, flame photometry, centrifugation, automated chemistry and haematology analysers, and the molecular platforms now used for PCR.
Core rules
- Spectrophotometry follows Beer–Lambert: absorbance is proportional to concentration when path length and wavelength are fixed. Choose the wavelength of maximum absorbance and zero the instrument with a blank.
- Cuvettes must be clean, scratch-free and matched; bubbles and fingerprints cause stray light and drift.
- Ion-selective electrodes measure activity, not concentration; they are sensitive to protein coating and to temperature.
- Flame photometry measures sodium and potassium by emission; lithium is used as an internal standard.
- Centrifuges should be set by relative centrifugal force, not revolutions per minute, because RCF depends on rotor radius.
- Automated analysers need calibration, daily quality control and maintenance; a result outside control limits is not reportable.
Worked example
A serum sample gives absorbance 0.30 in a glucose assay where a 5.0 mmol/L standard gives 0.25 with a 1 cm path length. Using proportionality, concentration = 0.30/0.25 × 5.0 = 6.0 mmol/L. If the cuvette had a scratch, absorbance would read high and the result would falsely rise, so repeat after visual inspection and re-zeroing.
Traps
Assuming a printed analyser result is automatically valid; calibrating without checking that the calibrator has not expired; and mixing cuvettes of different path lengths in one run.