Heparin Dosing Calculator
Calculate weight-based heparin drip rates and bolus doses with aPTT adjustments. Enter values for instant results with step-by-step formulas.
Formula
Bolus (units) = Weight (kg) x Bolus dose (units/kg); Infusion Rate (units/hr) = Weight (kg) x Rate (units/kg/hr); mL/hr = units/hr / (concentration units / volume mL)
The bolus dose is calculated by multiplying the patient weight by the protocol-specified bolus factor (typically 80 units/kg). The continuous infusion rate multiplies weight by the infusion factor (typically 18 units/kg/hr). The pump rate in mL/hr is derived by dividing the units/hr by the bag concentration in units/mL.
Worked Examples
Example 1: Standard Weight-Based Heparin Initiation
Problem: A 75 kg patient with a new DVT needs heparin anticoagulation. The pharmacy supplies 25,000 units in 500 mL D5W. Calculate the bolus dose and infusion rate.
Solution: Bolus: 75 kg x 80 units/kg = 6,000 units\nConcentration: 25,000 / 500 = 50 units/mL\nBolus volume: 6,000 / 50 = 120 mL\nInfusion: 75 kg x 18 units/kg/hr = 1,350 units/hr\nInfusion rate: 1,350 / 50 = 27 mL/hr\nDaily units: 1,350 x 24 = 32,400 units
Result: Bolus: 6,000 units (120 mL) | Infusion: 1,350 units/hr (27 mL/hr)
Example 2: aPTT Adjustment for Subtherapeutic Level
Problem: The same 75 kg patient has an aPTT of 38 seconds (below the 45-70 target range). Current rate is 1,350 units/hr. Adjust per protocol.
Solution: aPTT 35-45: Re-bolus 40 units/kg = 75 x 40 = 3,000 units\nIncrease rate by 2 units/kg/hr = 75 x 2 = 150 units/hr\nNew rate: 1,350 + 150 = 1,500 units/hr\nNew mL/hr: 1,500 / 50 = 30 mL/hr\nRecheck aPTT in 6 hours
Result: Re-bolus: 3,000 units | New rate: 1,500 units/hr (30 mL/hr)
Frequently Asked Questions
What is weight-based heparin dosing and why is it used?
Weight-based heparin dosing calculates the initial bolus and continuous infusion rate based on a patient's actual body weight in kilograms. This approach has been shown to achieve therapeutic anticoagulation faster than fixed-dose protocols, typically reaching target aPTT values within 24 hours rather than 48 to 72 hours. The standard weight-based protocol uses 80 units per kilogram for the initial bolus and 18 units per kilogram per hour for the continuous infusion. This method reduces the risk of both under-dosing, which can lead to clot extension, and over-dosing, which increases bleeding risk significantly.
What is the target aPTT range for heparin therapy?
The target activated partial thromboplastin time for heparin therapy is generally 1.5 to 2.5 times the control value, which typically corresponds to an aPTT of 45 to 70 seconds in most laboratories. However, the exact therapeutic range varies between institutions because different aPTT reagents and analyzers produce different results. Many hospitals calibrate their aPTT range to correspond to a heparin level of 0.3 to 0.7 units per milliliter by anti-factor Xa assay. It is essential to use the institution-specific nomogram rather than a generic range to ensure accurate dose adjustments and optimal patient safety.
What are the major complications of heparin therapy?
The most significant complication of heparin therapy is bleeding, which occurs in approximately 1 to 5 percent of patients receiving therapeutic doses. Heparin-induced thrombocytopenia (HIT) is another serious complication that develops in about 1 to 3 percent of patients, typically between days 5 and 14 of therapy, and paradoxically causes thrombosis rather than bleeding. Osteoporosis can occur with long-term heparin use exceeding several months. Hyperkalemia is an underappreciated side effect that occurs because heparin suppresses aldosterone synthesis. Monitoring platelet counts every 2 to 3 days during the first two weeks of therapy is recommended to detect HIT early.
What is the difference between unfractionated heparin and low molecular weight heparin?
Unfractionated heparin (UFH) has a variable molecular weight ranging from 3,000 to 30,000 daltons and works by binding antithrombin III to inhibit both thrombin and factor Xa. Low molecular weight heparins like enoxaparin have a more uniform molecular weight around 4,000 to 5,000 daltons and primarily inhibit factor Xa. UFH requires continuous intravenous infusion with frequent aPTT monitoring, while LMWH can be given as subcutaneous injections once or twice daily without routine monitoring. UFH is preferred in situations requiring rapid reversal with protamine, in renal failure where LMWH accumulates, and during cardiac surgery or dialysis procedures.
How does renal function affect heparin dosing?
Unfractionated heparin is primarily cleared through the reticuloendothelial system and does not rely heavily on renal elimination, making it safer than low molecular weight heparins in patients with impaired kidney function. Patients with a creatinine clearance below 30 milliliters per minute should generally receive UFH rather than LMWH because enoxaparin and similar agents accumulate and increase bleeding risk in renal impairment. However, even with UFH, patients with severe renal dysfunction may have altered pharmacokinetics and may require more frequent aPTT monitoring. Hepatic impairment can also affect heparin metabolism and antithrombin III production, potentially altering the dose-response relationship.
How should heparin be reversed in cases of bleeding?
Protamine sulfate is the specific antidote for heparin-induced bleeding and works by forming a stable complex with heparin molecules, neutralizing their anticoagulant effect. The dosing of protamine is based on the amount of heparin received, with 1 milligram of protamine neutralizing approximately 100 units of heparin. For continuous infusions, only the heparin administered in the preceding 2 to 3 hours needs to be considered because of heparin's short half-life of 60 to 90 minutes. Protamine should be administered slowly over 10 minutes because rapid infusion can cause hypotension, bradycardia, and anaphylactoid reactions. Maximum single dose should not exceed 50 milligrams.