Clinical Biochemistry
What this domain tests
Clinical biochemistry tests pre-analytical handling, common analytes, units, reference intervals and the effect of hemolysis or delayed separation on results.
Core rules
- Collect the correct tube: fluoride-oxalate for glucose, plain or gel tube for most chemistry, and EDTA for HbA1c and hematology. Fill citrate tubes to the marked volume.
- Fasting samples are usually taken after 8-12 hours for glucose and lipids. Avoid prolonged tourniquet use and fist clenching, which raise potassium and lactate.
- Separate serum or plasma promptly. Hemolysis falsely raises potassium, LDH and AST and interferes with many colorimetric methods.
- Know common patterns: high glucose with high HbA1c suggests diabetes; raised urea and creatinine suggest impaired renal function; raised ALT/AST suggests hepatocellular injury; raised troponin suggests myocardial injury.
- Report in SI units with the laboratory's reference interval, and state whether the sample was hemolyzed or lipemic.
Worked example
A serum sample is visibly pink and potassium is 6.9 mmol/L, while the patient is well and has normal ECG. Reasoning: hemolysis releases potassium from red cells, so the result is likely spurious. Recollect without prolonged tourniquet, separate promptly, and repeat before acting on the value.
Traps
- Reporting a hemolyzed potassium as a true hyperkalemia.
- Using an EDTA tube for chemistry, which raises potassium and lowers calcium.
- Delaying separation, which lowers glucose and raises potassium.
- Ignoring lipemia or icterus as interference.