How often should Partial Thromboplastin Time (PTT) be repeated for a heparin drip?

PTT Monitoring for Heparin Drip

Check PTT 4 hours after the initial heparin bolus and 4 hours after every infusion rate change until therapeutic levels are achieved, then monitor daily once stable. 1, 2

Initial Monitoring Protocol

• Draw the first PTT 4 hours after starting the heparin infusion to assess initial anticoagulation response and guide dose adjustments 1
• Measure PTT 6 hours after the initial bolus according to FDA labeling, though clinical guidelines favor the 4-hour timepoint for more rapid titration 2
• Continue checking PTT every 4 hours after each dose adjustment until the therapeutic range is achieved 1

Therapeutic Target Range

• Target PTT of 60-85 seconds, which corresponds to an anti-factor Xa level of 0.35-0.7 U/mL 1, 3
• For institutions with different baseline PTT values, adjust the target to 1.5-2.5 times the control value (e.g., if control PTT is 30 seconds, target 45-75 seconds) 1
• Never accept PTT <50 seconds as adequate—this carries a 15-fold increased risk of recurrent venous thromboembolism 3, 4

Maintenance Monitoring

• Once therapeutic PTT is achieved and stable, check PTT daily 1, 2
• Monitor platelet count at baseline, on day 5, then every 2-3 days throughout heparin therapy to detect heparin-induced thrombocytopenia 4, 2
• Perform periodic hematocrit and stool occult blood testing regardless of administration route 2

Critical Pitfalls to Avoid

• Subtherapeutic anticoagulation (PTT <50 seconds) increases thrombotic risk 15-fold—this is the most dangerous error in heparin management 3, 4
• PTT values of 50-59 seconds still carry increased thrombotic risk despite appearing "close" to therapeutic 3
• PTT >90 seconds increases bleeding risk without additional antithrombotic benefit and requires immediate dose reduction 3
• Delays in laboratory turnaround time cause prolonged periods of over- or under-anticoagulation—address this with your laboratory 1

When PTT Monitoring Fails

• Switch to anti-factor Xa monitoring (target 0.35-0.7 U/mL) if PTT results are unreliable or unobtainable 3, 4, 5
• In critically ill patients with hyperinflammatory states, elevated factor VIII and fibrinogen cause heparin resistance where PTT normalizes despite inadequate heparin effect—use anti-Xa assay instead 4
• Never hold therapeutic anticoagulation to obtain a laboratory value when active thrombosis is present—continue the infusion and troubleshoot specimen collection issues 5

Dose Adjustment Algorithm

When PTT results return, adjust according to established weight-based nomograms:

• PTT <35 seconds: Give 80 units/kg bolus, increase infusion by 4 units/kg/hour 3
• PTT 35-45 seconds: Give 40 units/kg bolus, increase infusion by 2 units/kg/hour 3
• PTT 46-70 seconds: No change needed 3
• PTT 71-90 seconds: Decrease infusion by 2 units/kg/hour 3
• PTT >90 seconds: Hold infusion for 1 hour, then decrease by 3 units/kg/hour 3

Real-World Performance Data

• Only 19-28% of patients achieve therapeutic PTT within the first 48 hours using standard dosing protocols 6
• No patient maintains all therapeutic PTT values throughout treatment, highlighting the difficulty of heparin management 6
• Use of standardized nomograms increases the proportion achieving therapeutic PTT at 24 hours from 37% to 66% 7

Special Populations

• Pediatric patients: Check PTT 4 hours after bolus and after each rate change, targeting 60-85 seconds (anti-Xa 0.35-0.70 U/mL) 1
• Patients requiring >35,000 units/24 hours: Consider switching to anti-factor Xa monitoring to avoid unnecessary dose escalation when PTT is blunted by elevated procoagulants 8
• Cardiovascular surgery: Follow institutional protocols for intraoperative monitoring during cardiopulmonary bypass 2