Falling platelets on a heparin infusion
A 64-year-old man is on day 5 of an unfractionated heparin infusion for a non-ST-elevation myocardial infarction. The bag contains 25,000 units of heparin in 250 mL of 0.9% sodium chloride; the prescription is 12 units/kg/hour and he weighs 82 kg. Today his platelet count is 96 × 10⁹/L, down from 268 × 10⁹/L on admission. His right forearm cannula site is tender and erythematous, and he reports new pain and swelling in the left calf. The aPTT is 2.9 times control against a target of 1.5–2.5. The junior doctor has just written 'reduce heparin to 10 units/kg/hour and repeat aPTT in 6 hours' and has asked you to sign the chart as checked. There is no documented heparin allergy. Pharmacy is closed and the haematology registrar is contactable by phone.
1. Calculate the currently prescribed infusion rate in mL/hour and state what you would verify before administering or altering any dose.
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Calculation
- Dose required: 12 units/kg/h × 82 kg = 984 units/hour.
- Concentration in the bag: 25,000 units ÷ 250 mL = 100 units/mL.
- Rate: 984 ÷ 100 = 9.84 mL/h, set as 9.8 mL/h on the pump.
Checks before touching the infusion
- Read the prescription against the bag label - units, total volume, concentration and expiry - because heparin bags come in several concentrations and a tenfold error is a classic never-event.
- Confirm patient identity, weight in kilograms (measured, not estimated) and the pump programming against the calculated rate.
- Confirm the most recent aPTT, platelet count and any bleeding.
- Inspect all cannula sites for haematoma or extravasation, and check for haematuria, melaena, epistaxis or new bruising.
- Never adjust the rate on a verbal instruction alone; the change must be written, signed and dated.
A rate of 9.8 mL/h is consistent with the original prescription, but that prescription is now clinically obsolete - the correct action is to question the whole plan rather than recalculate a dose that should not be given at all.
2. Interpret the platelet trend and the new limb findings, name the most likely diagnosis, and estimate a 4T score.
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Platelet trend: a fall from 268 to 96 × 10⁹/L is a 64% drop, and it has occurred between day 5 and day 6 of heparin exposure. That timing and magnitude are the classic signature of immune heparin-induced thrombocytopenia, not dilution, sepsis or drug-induced marrow suppression.
Limb findings: the tender, erythematous cannula site and the new painful swollen left calf point to thrombosis - superficial or catheter-related thrombosis and a probable deep vein thrombosis. HIT is a prothrombotic disorder; bleeding is not the expected complication, and that is exactly why the junior doctor's plan is dangerous.
Estimated 4T score
- Thrombocytopenia: fall >50% → 2
- Timing: onset day 5–10 → 2
- Thrombosis: new thrombosis present → 2
- Other causes: none evident → 0–1
Total approximately 6–7, indicating high probability. A score of 4 or more should prompt stopping heparin while confirmatory testing (PF4 antibody, then serotonin release assay) is arranged. The aPTT of 2.9 times control suggests over-anticoagulation but does not explain the platelet fall.
3. What do you do about the new prescription, and what must be avoided in this patient's ongoing management?
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Immediate action
- Do not sign or implement the revised prescription. Reducing the dose continues heparin exposure in a patient with probable HIT and active thrombosis.
- Stop the heparin infusion now and cap or remove the line; document the exact time.
- Phone the haematology registrar and the parent medical team, stating the platelet trend, the thrombotic symptoms and the calculated 4T score, and ask for a non-heparin anticoagulant to be prescribed (argatroban, bivalirudin or fondaparinux, chosen by renal and hepatic function and by whether invasive procedures are planned).
- Request urgent duplex ultrasound of the left leg and arrange review of the cannula site.
- Record heparin-induced thrombocytopenia as an allergy on the chart, the electronic record and the wristband, and hand this over in writing.
What must be avoided
- Platelet transfusion, unless there is life-threatening bleeding - it fuels the thrombotic process.
- Low-molecular-weight heparin and heparin flushes, which cross-react with HIT antibodies.
- Warfarin until platelets recover above 150 × 10⁹/L, because of the risk of venous limb gangrene and warfarin-induced skin necrosis.
- Any future heparin exposure, including during bypass or dialysis, without a documented plan. Denying or delaying the report to protect a colleague would be a serious professional failure; the incident must be recorded.