Medical Laboratory Technology: Specimen Collection, Handling and Safety
Domain overview
This domain tests specimen collection, labelling, transport and safety. Most laboratory errors begin before the sample reaches the bench, so the examiner rewards technique and identification.
Core rules and concepts
Identify the patient with at least two identifiers, explain the test, obtain consent where needed and follow standard precautions: hand hygiene, gloves and a tourniquet applied for less than one minute. Label the tube at the bedside, not later. Use the correct tube: lavender EDTA for haematology; light-blue 3.2% sodium citrate for coagulation; plain red or gold serum separator for chemistry; green heparin for some chemistry and blood gases; grey fluoride-oxalate for glucose.
Follow the order of draw to prevent carryover: blood culture first, then citrate, then serum tubes, then heparin, then EDTA, then fluoride-oxalate. Fill citrate tubes to the mark because the 9:1 blood-to-anticoagulant ratio matters. Mix by gentle inversion, never shake, to avoid haemolysis.
Timing matters: fasting glucose and lipids need 8–12 hours of fasting; blood cultures should be taken before antibiotics, usually two sets from different sites; urine is a clean-catch midstream sample, transported within two hours or refrigerated; sputum for AFB is best as an early-morning sample, with three samples on consecutive days; 24-hour urine must be collected completely and kept cool.
Worked example
A request asks for CBC, PT/INR and fasting glucose from one venepuncture. Draw citrate first, then EDTA, then fluoride-oxalate. If EDTA were drawn first, traces could carry over into the citrate tube, bind calcium and falsely prolong the INR. An underfilled citrate tube has the same effect. The glucose tube should be processed promptly because cells continue to consume glucose.
Traps that cost marks
Unlabelled or wrongly labelled tubes; drawing above a running IV line; underfilling citrate; vigorous shaking; delayed transport; and collecting urine in a non-sterile container for culture.