What is the management for a patient with subtherapeutic anticoagulation, as indicated by an Activated Clotting Time (ACT) of 16, on unfractionated heparin?

Management of Subtherapeutic Anticoagulation (ACT 16) on Unfractionated Heparin

For a 54-year-old male with an ACT of 16 seconds on unfractionated heparin, immediately administer an intravenous bolus of UFH (50-100 U/kg depending on clinical context) followed by increased continuous infusion, with repeat ACT measurement in 2-3 hours to verify therapeutic range has been achieved. 1

Understanding the Clinical Context

An ACT of 16 seconds is critically subtherapeutic and essentially represents no anticoagulation effect. The clinical indication for heparin determines the target ACT range:

• For PCI without GP IIb/IIIa inhibitors: Target ACT 250-300 seconds (HemoTec) or 300-350 seconds (Hemochron) 1
• For PCI with GP IIb/IIIa inhibitors: Target ACT 200-250 seconds 1
• For general anticoagulation (DVT/PE): ACT is not the preferred monitoring method; use aPTT with target ratio 1.5-2.5 1

Immediate Bolus Dosing

Administer weight-based UFH bolus immediately:

• Without planned GP IIb/IIIa inhibitors: 70-100 U/kg IV bolus 1
• With planned GP IIb/IIIa inhibitors: 50-70 U/kg IV bolus 1
• For non-cardiac arterial procedures: 100 U/kg achieves adequate ACT ≥200 seconds in 78% of patients 2

The 2013 ACCF/AHA guidelines explicitly recommend these weight-based boluses rather than fixed doses, as fixed 5,000 IU doses fail to achieve therapeutic anticoagulation in the majority of patients 1, 2

Continuous Infusion Adjustment

After the bolus, increase the continuous infusion rate:

• Standard initial infusion: Start or increase to 18 U/kg/hour (approximately 1,250-1,500 U/hour for average adult) 1
• Higher starting rates achieve therapeutic levels faster without increasing bleeding risk, provided the rate exceeds 1,250 U/hour 1
• For patients requiring large doses (>35,000 U/24 hours), consider anti-Xa monitoring rather than aPTT if available 3

Monitoring Strategy

Measure ACT 2-3 hours after bolus and infusion adjustment:

• ACT should be checked before any interventional procedure to confirm therapeutic anticoagulation 1
• For ongoing therapy, measure ACT or aPTT every 4-6 hours until stable in therapeutic range 1
• Once stable, daily monitoring is sufficient 1

Critical caveat: ACT measurements are device-dependent—HemoTec and Hemochron devices yield different values, so know which device your institution uses 1, 4

Special Considerations and Pitfalls

If ACT remains subtherapeutic despite high doses:

• Consider anti-Xa level monitoring (target 0.3-0.6 IU/mL) rather than continuing dose escalation based on ACT/aPTT alone 1, 3
• Subtherapeutic aPTT despite high heparin doses may reflect increased heparin-binding proteins rather than true inadequate anticoagulation 1
• In this scenario, switching to LMWH may be more effective as these agents have less protein binding 1

Avoid these common errors:

• Do not use ACT to monitor low-molecular-weight heparin—it is insensitive to LMWH even at therapeutic doses 5
• Do not rely on ACT alone in critically ill ICU patients, as multiple factors (antithrombin levels, platelet count, CRP, hematocrit) can affect ACT independent of heparin effect 4
• Do not give additional anticoagulants if the patient recently received a different agent (e.g., enoxaparin within 8 hours)—this increases bleeding risk 1

For patients on concurrent warfarin:

• Warfarin directly prolongs aPTT by approximately 20 seconds for each 1.0 increase in INR, which can lead to falsely "therapeutic" aPTT values despite subtherapeutic heparin levels 6
• In this situation, anti-Xa monitoring is superior to aPTT 6, 3

Procedural Timing

If PCI or intervention is imminent:

• Administer bolus immediately and proceed with intervention once ACT is verified in therapeutic range 1
• Do not delay urgent procedures for prolonged anticoagulation optimization 1
• For arterial sheath removal post-procedure, wait until ACT falls to 150-180 seconds to minimize bleeding risk 7