Incorrect Timing of aPTT Monitoring
The error described is incorrect timing of aPTT monitoring (Option A), because the aPTT was drawn at 2 hours after UFH initiation when guidelines clearly specify the first aPTT should be measured 4 to 6 hours after starting the heparin infusion. 1
Why This Represents a Timing Error
The European Society of Cardiology explicitly states that the first aPTT should be measured 4 to 6 hours after initiation of the heparin infusion, not at 2 hours 1
The American College of Cardiology/American Heart Association guidelines specify that measurements should be made 6 hours after any dosage change and used to adjust UFH infusion until the aPTT exhibits a therapeutic level 1
Drawing the aPTT at 2 hours does not allow sufficient time for the heparin infusion to reach steady-state pharmacokinetics, leading to a falsely subtherapeutic result 1
Understanding the Pharmacokinetic Rationale
Peak plasma concentration occurs immediately after IV bolus administration, but the continuous infusion requires 4-6 hours to achieve steady-state levels that accurately reflect the anticoagulant effect 2
The biphasic clearance of heparin (rapid saturable clearance followed by slower first-order elimination) means that early aPTT measurements do not reflect the true anticoagulant state 2
Premature aPTT testing at 2 hours will systematically underestimate the anticoagulant effect, leading to inappropriate dose escalation and potential over-anticoagulation 1
Why Other Options Are Incorrect
Option B (Failure to monitor efficacy) is incorrect because monitoring was actually performed—the problem was the timing, not the absence of monitoring 1
Option C (Failure to monitor adverse effects) is incorrect because aPTT monitoring is for efficacy assessment, not adverse effect monitoring; adverse effects like bleeding and thrombocytopenia require hemoglobin/hematocrit and platelet count monitoring 1, 3
Option D (Incorrect timing of UFH administration) is incorrect because there is no indication that the UFH was administered at the wrong time—the medication was given appropriately, but the monitoring occurred too early 1
Critical Clinical Pitfall to Avoid
Delays in laboratory turnaround time for aPTT results can be a source of variability in care, resulting in over- or under-anticoagulation for prolonged time periods, so institutions should establish protocols that account for both proper timing of draws AND rapid result reporting 1
When 2 consecutive aPTT values are therapeutic, measurements may be made every 24 hours, but premature initial testing disrupts this entire monitoring cascade 1
The nurse's adjustment based on a 2-hour aPTT likely led to unnecessary dose escalation, potentially increasing bleeding risk when the true steady-state level is reached 1, 3