Laboratory reasoning
Pre-analytical errors and safe test interpretation
Overview and approach
A result reflects sampling, processing and physiology; check validity before treating an unexpected number.
- Example: artefactual thrombocytopenia occurs in about 1 in 1000 results (platelet clumping, collection errors, satellitism), so an unexpected low platelet count warrants a repeat (1).
Assessment and clinical features
Timing, fasting, specimen type, haemolysis, tourniquet, line contamination, medicines and clinical fit.
- EDTA (FBC tube) can cause in-vitro platelet clumping; citrate and heparin do not (1).
- Some groups have different normal values: people of African and Middle Eastern origin often have a neutrophil count of 1.0 to 1.5 × 10^9/L (2).
Differentials and investigations
Compare trends and units/reference intervals; repeat suspicious values if safe and discuss with laboratory staff.
- Review previous results: a low count may be normal for the patient (1). A repeat sample in citrate, or a blood film, confirms pseudothrombocytopenia (1).
Management and follow-up
Order tests that answer a clinical question and document action/follow-up of abnormal results.
- Laboratories should phone positive blood cultures and critical results to the treating doctor and document the discussion (3).
Red flags and clinical pearls
Never delay treatment of a plausible critical abnormality solely to repeat it; also avoid treating a contaminated sample without assessment.
Last updated from Sen’s notes on 9 October 2026.