How is weight-based heparin dosing calculated?

For DVT/PE treatment, the standard weight-based protocol uses an initial IV bolus of 80 units/kg followed by a continuous infusion of 18 units/kg/hr. For ACS, the bolus is 60 units/kg (max 4,000 units) with an infusion of 12 units/kg/hr (max 1,000 units/hr). Doses are based on actual body weight, though some protocols cap weight at 150 kg for obese patients.

How do you adjust heparin based on aPTT?

The Raschke nomogram adjusts heparin based on aPTT results: aPTT 90s — hold infusion for 1 hour and decrease by 3 u/kg/hr.

What is the therapeutic aPTT range for heparin?

The commonly targeted therapeutic aPTT range is 46–70 seconds, which typically corresponds to an anti-Xa level of 0.3–0.7 IU/mL. However, aPTT ranges vary by laboratory reagent and instrument, so institutions should calibrate their therapeutic range to correspond to therapeutic anti-Xa heparin levels.

How often should aPTT be monitored on a heparin drip?

Check aPTT every 6 hours after initiation or any dose change. Once two consecutive aPTT values are in the therapeutic range, monitoring can be spaced to every 24 hours. If the dose is adjusted, restart the 6-hour monitoring cycle.

What concentration is used for heparin infusion?

The standard premixed heparin concentration is 25,000 units in 250 mL (D5W or NS), yielding 100 units/mL. This allows straightforward conversion from units/hr to mL/hr by dividing by 100. Some institutions use 25,000 units in 500 mL (50 units/mL) — always verify your institution's standard concentration.

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Heparin Dosing Calculator

Weight-based IV heparin dosing with bolus, infusion rate, and aPTT adjustment nomogram.

Initial Dosing

Indication

Patient Weight

Cap dosing weight at 150 kg (for obese patients)

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Enter weight to calculate heparin dosing.

aPTT Dose Adjustment

Enter aPTT result to get dose adjustment per the Raschke nomogram.

Current aPTT (seconds)

Current infusion rate (units/hr)

aPTT Nomogram Reference

aPTT (seconds)	Re-Bolus	Rate Change	Hold
< 35	80 u/kg	↑ Increase by 4 u/kg/hr	No
35–45	40 u/kg	↑ Increase by 2 u/kg/hr	No
46–70	None	No change (therapeutic)	No
71–90	None	↓ Decrease by 2 u/kg/hr	No
> 90	None	↓ Decrease by 3 u/kg/hr	Hold 1 hour

Disclaimer: For educational purposes only. Not a substitute for clinical judgment. Always verify with your institution's heparin protocol and pharmacy guidelines.

How it's used

DVT/PE: Bolus 80 u/kg IV, then infuse 18 u/kg/hr. No max dose caps in standard protocol.
ACS: Bolus 60 u/kg IV (max 4,000 u), then 12 u/kg/hr (max 1,000 u/hr) per ACC/AHA.
VTE Prophylaxis: 5,000 units SC every 8–12 hours (fixed dose, no weight calculation).
Standard bag: 25,000 units in 250 mL = 100 u/mL. Divide units/hr by 100 to get mL/hr.
Check aPTT 6 hours after any bolus or rate change. Two therapeutic values → q24h monitoring.

Weight considerations

Use actual body weight for standard patients.
For BMI > 40 or weight > 150 kg, consider capping dosing weight at 150 kg (protocol-dependent).
Adjusted body weight = IBW + 0.4 × (actual − IBW) is an alternative used by some institutions.

Monitoring schedule

aPTT every 6 hours until 2 consecutive values in therapeutic range (46–70 s).
Then aPTT every 24 hours.
Platelet count at baseline and every 2–3 days for HIT surveillance.
Signs of bleeding: guaiac stools, hemoglobin drop, petechiae.

References

Raschke RA, et al. The weight-based heparin dosing nomogram compared with a "standard care" nomogram. Ann Intern Med. 1993;119(9):874–881.
Kearon C, et al. Antithrombotic therapy for VTE disease: CHEST guideline and expert panel report. Chest. 2016;149(2):315–352.
UpToDate. Heparin dosing and monitoring.
Amsterdam EA, et al. 2014 AHA/ACC guideline for the management of patients with NSTE-ACS. Circulation. 2014;130(25):e344–e426.

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About This Tool

What Is the Weight-Based Heparin Dosing Protocol?

The weight-based heparin dosing protocol, originally validated by Raschke et al. in 1993, calculates initial IV heparin bolus and continuous infusion rates based on patient weight. This approach achieves therapeutic anticoagulation faster and more reliably than fixed-dose or empiric nomograms. The protocol includes a structured aPTT-based dose adjustment nomogram to titrate the infusion to a target therapeutic range.

When to Use Weight-Based Heparin Dosing

Weight-based IV heparin is indicated for acute treatment of venous thromboembolism (DVT and PE), acute coronary syndromes (unstable angina, NSTEMI), and other conditions requiring rapid therapeutic anticoagulation such as bridging for mechanical heart valves. For VTE prophylaxis in hospitalized patients, a fixed subcutaneous dose of 5,000 units every 8–12 hours is used instead.

DVT/PE vs ACS Protocols

The DVT/PE protocol uses higher initial doses (bolus 80 u/kg, drip 18 u/kg/hr) because the therapeutic target requires higher levels of anticoagulation. The ACS protocol uses lower doses (bolus 60 u/kg max 4,000 u, drip 12 u/kg/hr max 1,000 u/hr) because patients receive concurrent antiplatelet therapy and the bleeding risk is managed differently. Always verify which protocol your institution uses.

Obesity and Dosing Weight Adjustments

In obese patients (BMI > 40), using actual body weight may lead to supratherapeutic levels and increased bleeding risk. Some institutions cap dosing weight at 150 kg; others use adjusted body weight (IBW + 0.4 × excess). There is no universal consensus — follow your institutional protocol and monitor aPTT closely in this population.

🔑 Clinical Pearls

Weight-based dosing reaches therapeutic aPTT in ~24 hours vs ~48+ hours with empiric dosing.
Always confirm the heparin bag concentration — the most common is 25,000 u/250 mL (100 u/mL), but 25,000 u/500 mL (50 u/mL) is also used.
aPTT reagent sensitivity varies by lab — institutions should calibrate their therapeutic range to anti-Xa 0.3–0.7 IU/mL.
Hold heparin and recheck aPTT if patient develops signs of bleeding, thrombocytopenia (HIT concern), or aPTT > 90s.
Transition to warfarin or DOAC once clinically appropriate. Overlap heparin with warfarin for ≥5 days and until INR ≥ 2.0 for ≥24h.

Key References

Raschke RA, Reilly BM, Guidry JR, et al. The weight-based heparin dosing nomogram compared with a "standard care" nomogram. Ann Intern Med. 1993;119(9):874–881.
Kearon C, Akl EA, Ornelas J, et al. Antithrombotic therapy for VTE disease: CHEST guideline. Chest. 2016;149(2):315–352.
Garcia DA, Baglin TP, Weitz JI, Samama MM. Parenteral anticoagulants: ACCP evidence-based clinical practice guidelines. Chest. 2012;141(2 Suppl):e24S–e43S.

Formula last verified: February 2026