A Preoperative Coagulation Result That Does Not Fit the Patient
A 72-year-old woman is scheduled for elective knee arthroplasty. She takes no anticoagulants, has no personal or family history of bleeding, and has had two prior surgeries without excessive bleeding. Preoperative blood is drawn by a phlebotomist who obtains it from a 22-gauge peripheral intravenous catheter in the left hand, after 10 mL of saline was used to flush the line. The light blue citrate tube is a 2.7 mL draw tube and appears to contain roughly 1.3 mL of blood. Results: PT 19.8 seconds (reference 11.5 to 14.5), aPTT 48 seconds (reference 25 to 35), platelets 240 x 10^9/L, fibrinogen 320 mg/dL. The right antecubital area has a large hematoma from an earlier unsuccessful attempt. The surgeon asks whether to delay the case.
1. What is the most likely cause of these results, and how do the numbers support your answer?
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The tube was underfilled and drawn from a line, which is a double hit against the result.
Citrate tubes are calibrated for a fixed blood-to-citrate ratio. When the tube is short-drawn, the volume of blood is low relative to the fixed amount of liquid citrate, so the plasma is over-anticoagulated and excess citrate binds the calcium that the assay adds back. This prolongs clotting times in a way that is proportional to the shortfall. Filling roughly 1.3 mL of a 2.7 mL tube is about half the required volume, which produces a large artifact.
Saline flush that was not cleared from the line dilutes the sample further, and line draws can also carry traces of heparin from a locked catheter.
The pattern supports an artifact rather than a real coagulopathy: PT and aPTT are both moderately prolonged, fibrinogen is normal at 320 mg/dL, the platelet count is normal, and this patient has no bleeding history at all. Real coagulopathy causing prolongation of this degree would usually come with a suggestive history or an abnormal fibrinogen. It is the discordance between the result and the patient that matters.
2. What should be done now, and how would you communicate it to the surgeon?
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Reject the specimen and recollect before making any decision about the surgery. Do not report a critical-range coagulation result from an underfilled line draw, even though it is the only sample available and the case is scheduled.
Collection plan for a patient with a hematoma in the other arm:
- Use a 21-gauge needle and a straight needle-and-holder system if possible; a 22-gauge catheter is the wrong device for a coagulation sample.
- If a line must be used, stop any infusion, flush with saline, discard the first 5 to 10 mL, and then fill the citrate tube to the line. Never fill a citrate tube from a line that has been used for heparinized fluids without a substantial discard.
- Allow the tube to fill to the indicated volume; do not pull the plunger or pop the tube off early.
- Label the specimen with the collection site and note that it is a line draw.
Communicate plainly: the current result is invalid due to a specimen collection problem, the patient has no bleeding history, and the test will be repeated. If a properly collected sample is still prolonged, that is the point at which to order a mixing study and involve hematology, and a delay of the case becomes a real discussion.
3. Which additional tests would you consider vulnerable to the same collection problems, and what acceptance criteria should the laboratory enforce?
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Any test whose result depends on the concentration of an additive or on the diluent effect is vulnerable:
- Coagulation tests (PT, aPTT, fibrinogen, D-dimer, anti-Xa) are the classic casualties because citrate is the additive.
- Blood gases and lactate are distorted by saline dilution and by line flush.
- Blood cultures drawn through a line without proper disinfection and discard grow contaminants rather than pathogens.
- Routine chemistry results from a line draw are also suspect if the discard volume was inadequate, because dilution lowers everything uniformly.
Acceptance criteria to enforce:
- Citrate tubes must be filled to at least 90 percent of the nominal draw volume, and a short-draw tube should be rejected even if it is the only sample. A request to run it anyway should be documented and declined.
- The tube must be filled to the etched line, not merely partially filled.
- Note the collection source and any line draw on the requisition.
For this patient the follow-up matters more than the rule: after a correct draw, a normal result lets the surgery proceed, and the episode should be fed back to phlebotomy as a teaching case rather than simply repeated.