Sample Collection, Handling, Labelling and Storage
Pre-analytical error causes most laboratory mistakes, so candidates must understand the full chain: patient or source identification, correct collection container and additive, correct volume, timely transport, correct storage and unambiguous labelling. Know that blood tubes have colour-coded tops reflecting additives: EDTA (purple) for haematology, citrate (light blue) for coagulation, serum separator (gold or red) for chemistry, fluoride oxalate (grey) for glucose. Order of draw matters to avoid additive carryover. Know that some samples require ice, some require protection from light, and some must be centrifuged within a set time.
Traps: labelling the tube before collection at the bedside is a patient safety breach; labelling the lid rather than the tube is unacceptable; a haemolysed or clotted sample is usually rejected; a 24-hour urine collection without correct preservative or total volume is invalid. Understand chain of custody for forensic or legal samples, and the meaning of primary versus secondary containers. Know rejection criteria and the importance of documenting them.
Worked example: A glucose sample arrives three hours after collection in a plain tube. Correct action is to reject it and request recollection, because glycolysis will have lowered the result; a fluoride oxalate tube inhibits glycolysis and is the appropriate container.
Rule of thumb: label at the point of collection, in the presence of the source, never in advance and never on the lid alone.