Phlebotomy
Phlebotomy questions focus on preanalytical error, because most laboratory mistakes begin at collection. You are expected to know tube additives and order of draw, acceptable technique, rejection criteria, and how a collection error corrupts a specific result.
Order of draw (CLSI)
- Blood culture bottles (sterile specimens) first, then sodium citrate (light blue), then serum tubes with or without clot activator and gel (red, gold), then sodium heparin (green), then EDTA (lavender), then sodium fluoride/potassium oxalate (gray).
- The sequence exists to prevent additive carryover: citrate into a serum tube or EDTA into a citrate tube changes calcium-dependent results.
- When blood cultures are ordered, fill them before other tubes; when only a citrate tube is drawn through a butterfly, draw a discard tube first to eliminate air in the tubing.
Technique and safety
- Tourniquet time should not exceed about 1 minute; prolonged application raises potassium, protein and cell counts by hemoconcentration.
- Use the appropriate gauge; small-bore needles and forceful aspiration lyse cells and falsely raise potassium, LDH and AST.
- Identify the patient by two identifiers and label at the bedside. Follow standard precautions, sharps disposal, and the bloodborne pathogen standard.
- Hemolyzed, clotted, short-draw, or mislabeled specimens are rejected and redrawn.
Special collections
- Capillary collections: wipe away the first drop, avoid squeezing, and warm the site before fingerstick glucose.
- Glucose tolerance testing requires fasting baseline and timed specimens in fluoride tubes.
- Avoid drawing above an infusing IV line; draw from the opposite arm when possible.
- Therapeutic drug monitoring requires correctly timed specimens, usually a trough before the next dose.
Worked example
A phlebotomist draws a lavender EDTA tube, then a light blue citrate tube, then a serum tube, and the coagulation results return as PT 22 s and aPTT 65 s with potassium 6.8 mmol/L. Reasoning: the EDTA tube was drawn first, so EDTA carried over into the citrate tube, chelating calcium and prolonging both clotting times while contaminating the potassium result. The EDTA also explains the pattern of abnormal results in a patient with no bleeding. The correct action is to recollect in the proper order, beginning with the citrate tube, and to report the event as a preanalytical error.
Traps that cost marks
- Drawing citrate after EDTA, which invalidates coagulation results.
- Underfilling citrate tubes and accepting the result anyway.
- Warming or squeezing a capillary site to obtain blood.
- Labeling tubes away from the patient.