Expansion
Most abnormal results are explained before the sample reaches the analyser
Expansion
Pre-analytical errors, much the commonest source of a spurious result
- Haemolysis: falsely raises potassium, LDH, AST, magnesium and phosphate, and lowers sodium by dilution
- Prolonged tourniquet or fist clenching: raises potassium, calcium and protein
- Delayed transport: raises potassium (cells leak) and lowers glucose (cells consume it), which is why glucose goes into a fluoride oxalate tube
- Wrong tube or wrong order of draw: EDTA contamination gives a very high potassium with a very low calcium, a classic and unmistakable pattern
- Drip arm sampling: dilutes everything except the constituent being infused
- Posture: standing raises protein bound analytes by about 10 per cent
- Fasting status: affects glucose and triglycerides
Analytical interference
- Biotin supplements interfere with many immunoassays, causing falsely high or low results including troponin and thyroid function
- Paraproteins, lipaemia, icterus and heterophile antibodies
Physiological variation
- Diurnal: cortisol highest on waking, iron and testosterone in the morning
- Postural, postprandial, and the menstrual cycle
- Pregnancy: raised alkaline phosphatase (placental), lowered albumin, urea and creatinine
“If the result does not fit the patient, repeat it before acting on it.” That single rule prevents most of the harm, and applies particularly to an unexpected hyperkalaemia in a well patient.