Specimen Collection and Pre-analytical Processes: Order of Draw and Stability
Pre-analytical error accounts for most laboratory mistakes, and this domain is examined heavily. You will be asked to identify the correct tube, the correct order of draw, and the effect of a suboptimal collection on the final result.
Core rules
- Order of draw per CLSI GP41: blood culture bottles, then sodium citrate (light blue), serum tube with or without clot activator/gel (red or gold), heparin (green), EDTA (lavender), and sodium fluoride/potassium oxalate (grey). The sequence prevents additive carry-over into tubes whose tests are additive-sensitive.
- Coagulation tubes contain 3.2% buffered sodium citrate at a 9:1 blood-to-anticoagulant ratio. They must be filled to at least 90% of the nominal volume; short draws give falsely prolonged PT and aPTT. When using a butterfly set with a discard tube, draw a non-additive tube first.
- EDTA binds calcium, so it is unsuitable for calcium, magnesium and most chemistry tests, and potassium EDTA falsely raises potassium. Heparin is the preferred tube for plasma electrolytes when a rapid turnaround is needed.
- Timed and diurnal collections matter: cortisol peaks around 08:00, therapeutic drug troughs are drawn just before the next dose, and a 24-hour urine requires a creatinine measurement plus volume to confirm completeness.
- Transport: most chemistry analytes are stable at room temperature for hours, but ammonia, lactate, blood gas, renin and ACTH require ice and rapid delivery; light protection is needed for bilirubin and porphyrins.
Worked example
A patient has potassium 6.9 mmol/L, calcium 1.52 mmol/L, magnesium 0.45 mmol/L and alkaline phosphatase 25 U/L, all on a serum tube, with a normal creatinine and ECG. Reasoning: the pattern of high potassium with low calcium, magnesium and ALP is the signature of potassium EDTA contamination, usually from drawing blood into the EDTA tube and then filling the serum tube from the same needle, or by backflow from a lavender tube. Potassium EDTA chelates divalent cations and delivers a large potassium load. The correct action is to recollect in the proper order of draw with a fresh needle, and to notify the clinician that the result is spurious.
Traps that cost marks
- Labelling tubes before the patient is identified; identification must occur at the bedside.
- Missing the fill line on citrate tubes, or collecting more than the recommended 1 mL minimum on small-volume paediatric tubes without adjusting for additive dilution.
- Prolonged tourniquet time or fist pumping, which raises potassium and protein.
- Failing to protect bilirubin from light.