Instrumentation and Equipment
Know the operating principles of major analyzers. Spectrophotometry relies on Beer-Lambert law (A = εbc); deviations occur at high absorbance or with stray light. Turbidimetry measures light blocked by particles; nephelometry measures scattered light and is more sensitive for low concentrations (e.g., specific proteins).
Electrolyte measurement: ion-selective electrodes (direct versus indirect) differ in undiluted versus diluted samples; pseudohyponatremia occurs with indirect ISE in hyperlipidemia or hyperproteinemia. Flame photometry and osmometry (freezing point depression, vapor pressure) are alternatives.
Chromatography (HPLC, gas, liquid) separates analytes; mass spectrometry provides definitive identification and is used for drugs, steroids, and newborn screening. Immunoassays (competitive versus sandwich) have hook effects at very high concentrations. Point-of-care devices and their limitations appear in later domains.
Automation concepts: random access versus batch, carryover, and throughput. Maintenance and calibration frequency, linearity, and the effect of reagent lot changes are tested. Understand the difference between calibration and calibration verification.
Worked example: a sodium of 118 mmol/L by indirect ISE in a patient with markedly elevated triglycerides should raise suspicion of pseudohyponatremia; direct ISE or measurement of serum osmolality clarifies.
Trap: assuming nephelometry and turbidimetry are interchangeable-nephelometry is superior at low antigen excess.