Chemistry
Measurement principles
Photometry follows Beer's law (A = ebc); absorbance is linear only inside the validated range, so results above the linear limit must be diluted and re-assayed. Endpoint reactions measure total absorbance, while kinetic (rate) methods measure change over time and are less affected by sample blank interference.
Quality control
Run at least two levels daily. Westgard rules: 1-2s is a warning; 1-3s, 2-2s, R-4s, 4-1s, and 10x are rejection rules.
Calculations worth memorizing
- Anion gap = Na - (Cl + HCO3), reference about 8-12 mmol/L (add roughly 2.5 if potassium is included).
- Calculated osmolality = 2Na + glucose/18 + BUN/2.8; a measured-minus-calculated gap above 10 suggests methanol, ethylene glycol, or isopropanol.
- Corrected calcium = measured Ca + 0.8 x (4.0 - albumin in g/dL).
- Corrected sodium in hyperglycemia = measured Na + 0.016 x (glucose - 100).
Traps
- Hemolysis falsely elevates potassium, LDH, AST, and phosphate.
- Lipemia and high bilirubin interfere on many platforms.
- Jaffe creatinine is nonspecific (ketones, protein, some drugs).
- Severe hyperglycemia or hyperproteinemia causes pseudohyponatremia on indirect ion-selective electrode systems.
- Troponin I/T is the preferred cardiac marker; CK-MB has largely fallen out of use.
- HbA1c reflects roughly 2-3 months of glycemia and is unreliable in hemoglobinopathy or after recent transfusion.
Rule of thumb
pH = 6.1 + log([HCO3] / (0.03 x pCO2)). In acute respiratory acidosis, bicarbonate rises only about 1 mmol/L per 10 mmHg pCO2; larger changes imply metabolic compensation.