Clinical Laboratory Testing and Analysis
Analytical principles
Most chemistry assays use spectrophotometry, ion-selective electrodes, or immunoassay; hematology uses impedance, light scatter, and flow cytometry; microbiology uses culture, staining, and molecular amplification. A reference interval is usually the central 95% of a healthy population, not a treatment threshold. Critical values are action limits that require immediate notification and documentation under the laboratory policy.
Interference and interpretation
Hemolysis falsely raises potassium, LDH, and AST and can bias colorimetric assays. Lipemia and icterus scatter light or absorb at the measurement wavelength. Immunoassays can be affected by heterophile antibodies, biotin, or the hook effect, in which extremely high analyte saturates both capture and detection antibodies and produces a falsely low result; dilution reveals the true value. Correct total calcium for albumin: corrected Ca = measured Ca + 0.8 × (4.0 − albumin in g/dL).
Worked example
A patient has total calcium 8.0 mg/dL and albumin 2.0 g/dL. Corrected Ca = 8.0 + 0.8 × (4.0 − 2.0) = 9.6 mg/dL. The low total calcium is explained by hypoalbuminemia, and ionized calcium is the physiologically active fraction to check if symptoms are present. Reporting the uncorrected value as true hypocalcemia would be an interpretation error.
Traps
Do not confuse analytical sensitivity (lowest detectable amount) with clinical sensitivity (ability to identify disease). A result inside the reference interval can still be critical for a given patient, and a critical value must be phoned even if it looks like a known chronic condition. Watch unit conversions and dilution factors; a result multiplied by the wrong dilution is a tenfold error.