The 23-Gauge Butterfly, the Short Citrate Tube, and a Biopsy at 14:00
A 71-year-old woman attends the emergency department with confusion and is booked for a liver biopsy at 14:00. A phlebotomist uses a 23-gauge butterfly in the dorsum of the hand and draws, in this order, a 2.0 mL K2EDTA tube (filled to about 0.7 mL), a 2.7 mL sodium citrate tube for PT/APTT (filled to about 1.3 mL) and a 4 mL lithium heparin tube, the last one transferred from a 5 mL syringe. No discard tube is taken. The EDTA sample reaches the laboratory 3 hours after collection: Hb 11.2 g/dL, RBC 3.9 ×10^12/L, MCV 92 fL, platelets 210 ×10^9/L. The citrate sample gives PT 16.8 s and APTT 46 s; two months earlier, drawn with a 21-gauge needle, her PT was 12.1 s and APTT 30 s (reference 11–14 s and 26–36 s). Plasma potassium in the heparin tube is 5.9 mmol/L and the hemolysis index is not raised. Her hematocrit is 52%. The clinician telephones: "The biopsy is at 14:00 - can I use these clotting results?"
1. Decide whether each of the three tubes should be accepted or rejected, explaining the preanalytical mechanism that makes the PT/APTT unreliable here.
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All three tubes should be rejected, but for different reasons.
EDTA tube. A 2.0 mL draw tube filled to 0.7 mL contains roughly three times the intended K2EDTA concentration. Cell sizing and platelet counting in the presence of excess EDTA are not validated, and the direction of bias is method-dependent, so the indices cannot be corrected arithmetically. The 3-hour delay is acceptable for a CBC on EDTA, but the short draw is not.
Citrate tube. A 2.7 mL tube contains about 0.3 mL of 3.2% citrate and needs about 2.4 mL of blood (1:8). At 1.3 mL the ratio is roughly 1:4.3, so there is excess citrate relative to plasma: it chelates ionized calcium and prolongs both PT and APTT. Worse, the citrate tube was drawn immediately after the EDTA tube through butterfly tubing with no discard; the dead space (roughly 0.2–0.3 mL) holds K2EDTA, which is flushed into the citrate tube and chelates calcium a second time. Both errors push in the same direction, so the 16.8 s/46 s cannot be interpreted as a real coagulopathy.
Heparin tube. A potassium of 5.9 mmol/L without hemolysis should raise suspicion of K2EDTA contamination (EDTA adds potassium and lowers calcium and magnesium). Because the sample was transferred from a syringe, delayed separation and cell leakage are also possible. It should be recollected and, on the same tube, calcium and magnesium checked.
The hematocrit of 52% does not require citrate-volume adjustment (that applies above about 55%), although a higher hematocrit reduces plasma volume and adds a small further excess of citrate relative to plasma.
2. What recollection plan and instructions would you give, and what would you tell the clinician about the 14:00 biopsy?
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Instructions to the collector: use a 21-gauge needle and a fresh venipuncture, ideally not the hand vein used before. If a butterfly set is unavoidable, draw and discard 2–3 mL first to clear the tubing dead space. Order of draw: citrate first, then serum or heparin, then EDTA. Fill the citrate tube to its fill line (about 2.4 mL in a 2.7 mL tube), invert gently three to four times, label with the collection time, and send at room temperature. Avoid syringe transfer unless the transfer is immediate and the tube is filled to the line; never top up a short tube from another tube.
What to tell the clinician: the current PT/APTT are preanalytically invalid and cannot be used to clear or cancel the biopsy. The combination of prior normal results, an underfilled citrate tube, and EDTA carryover fully explains the prolongation, but that is an inference, not a measurement. Repeat results should be available within about an hour. A fibrinogen and platelet count on the fresh sample will support the interpretation. If the patient is actively bleeding, management should follow the clinical picture rather than a sample known to be invalid; if she is stable, the biopsy can be rescheduled after the repeat. Do not simply release a comment saying "prolonged" - the report should state that the sample was rejected.
3. The fresh sample gives PT 12.3 s, APTT 31 s, potassium 4.1 mmol/L, calcium 2.35 mmol/L and magnesium 0.85 mmol/L. Explain the discrepancy and what you would document.
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The fresh results are consistent with the preanalytical explanation: the PT and APTT return to the patient's previous baseline, and the normal potassium, calcium and magnesium exclude clinically significant EDTA contamination. The original PT 16.8 s, APTT 46 s and potassium 5.9 mmol/L were therefore artefacts of an underfilled citrate tube, EDTA carryover from the preceding tube, and a heparin sample drawn through the same contaminated line.
Documentation should include: the rejection of all three original tubes with the reason for each; the repeat results; notification of the clinician and the time of notification; and an incident or nonconformance record so that the short-draw and order-of-draw problems can be reviewed with the collector. A comment on the final report noting that the earlier sample was rejected for quantity not sufficient is appropriate.
The generalisable points are that (a) two independent preanalytical errors can produce the same apparent coagulopathy, so correcting only one would not have fixed the result; (b) an unexpected potassium with a normal hemolysis index should prompt a check for EDTA contamination, particularly when an EDTA tube was drawn first; and (c) the biopsy decision needed a valid result, not a statistical guess.