Preoperative draw: an INR of 3.4 in a patient on no anticoagulant
A 42-year-old man attends an outpatient draw station for a preoperative workup: CBC, PT/INR, comprehensive metabolic panel, and two sets of blood cultures. His veins are small. The phlebotomist uses a 21-gauge winged set with a holder and collects in this sequence: light-blue citrate tube, which stops filling at 2.1 mL in a 2.7 mL tube when the line collapses; then a 4 mL EDTA tube; then a 5 mL serum separator tube; then the culture bottles, all from the same site. Yesterday's results were INR 1.1, potassium 4.1 mmol/L, calcium 9.4 mg/dL. Today: INR 3.4, potassium 6.8 mmol/L, calcium 6.2 mg/dL, magnesium 2.9 mg/dL, sodium 138 mmol/L. The patient reports no bleeding and takes no warfarin or other anticoagulant.
1. The coagulation and chemistry results are both inconsistent with this patient's history. Explain what most likely happened, citing the specific numbers.
Read the worked answer
Two independent preanalytical failures are superimposed.
Coagulation. 2.1 mL in a 2.7 mL tube is 78% of nominal fill; CLSI requires at least 90%. An underfilled citrate tube leaves excess citrate relative to the calcium in the plasma, so less ionized calcium is available to drive the cascade and the PT/INR is falsely prolonged. An INR of 3.4 in a patient on no anticoagulant, with a normal INR 24 hours earlier, should never be accepted. The winged set makes this worse: the tubing dead space displaces blood before the tube fills, so a discard tube is needed.
Chemistry. EDTA is a potassium salt and a calcium/magnesium chelator. Carryover into the serum tube produces exactly this triad: K 6.8 (was 4.1), Ca 6.2 (was 9.4), Mg 2.9, with a normal sodium. Isolated hyperkalemia with hypocalcemia and hypermagnesemia in a serum tube is the fingerprint of EDTA contamination.
2. Which of today's results may be released, and what must happen before anything reaches the chart?
Read the worked answer
Hold the INR, potassium, calcium, and magnesium. None of them reflects the patient and releasing an INR of 3.4 could trigger unnecessary vitamin K, plasma, or a cancelled operation.
Sodium is not affected by K2-EDTA, but the safest practice is to hold the whole panel and recollect. If a value is genuinely urgent, release it only with a comment stating the specimen was compromised and a redraw has been requested; that is a clinical decision made with the ordering provider, not a silent correction.
The blood cultures were drawn last, after several tubes from the same venipuncture, which raises the contamination risk; incubate them but flag the collection sequence so a single positive bottle is interpreted cautiously.
Actions: telephone the provider, document the event and the notification, redraw both arms (coagulation from a fresh, properly filled tube), and repeat the chemistry.
3. List the specific changes to collection technique that would have prevented this.
Read the worked answer
- Order of draw. Cultures first, then the coagulation tube, then serum, then EDTA. Drawing EDTA before serum is what contaminated the chemistry.
- Discard tube with a winged set. Draw a non-additive discard tube first so the tubing dead space is filled before the citrate tube.
- Correct tube size, not a partial tube. With small veins use a 2.7 mL blue top but let it fill until the vacuum is exhausted; a 21-gauge needle and a collapsing vein are a sizing problem, not a reason to submit a short draw.
- Never top up, pour, or transfer blood between tubes.
- Order-of-draw prompts on the bench for staff covering the outpatient station, plus a hard stop: an INR more than about 1.5 above a recent baseline should generate a call to the floor before reporting.