Laboratory Instrumentation, Analytical Principles, and Quality Control
Instrumentation, Analytical Principles, and Quality Control tests how analyzers measure signals, how methods are validated, and how statistical QC detects error. Expect calculations, calibration interpretation, and interference questions.
Core rules
- Spectrophotometers use a light source, monochromator, cuvette, and detector. Absorbance is proportional to concentration within the linear range.
- Calibration establishes the relationship between signal and concentration. Linearity must be verified; a high R² alone does not prove acceptable linearity across the clinical range.
- Precision is agreement among replicate measurements; accuracy is agreement with the true value. Bias is systematic difference; imprecision is random variation.
- Carryover occurs when a high sample contaminates the next sample; run blanks or wash cycles. Drift is progressive change over a run.
- QC statistics include mean, SD, and CV. Levey-Jennings charts display trend, shift, and random error.
Worked example
A glucose standard of 100 mg/dL gives absorbance 0.400. A patient sample gives absorbance 0.200 under the same conditions. Using proportional Beer–Lambert behavior, concentration = (0.200/0.400) × 100 = 50 mg/dL. Reasoning: absorbance is directly proportional to concentration within the linear range. If the sample absorbance were outside the calibrated range, dilute and re-run rather than extrapolate.
Traps
A wrong blank causes falsely high or low absorbance. Bubbles, dirty cuvettes, and lamp drift affect results. Do not use a calibration curve beyond its verified linear range. Do not accept QC based on a single point; evaluate trends and rules. Carryover can mimic a falsely elevated blank or a falsely low next result.