Heparin is a drug that thins the blood and prevents clotting. The amount of a dose of heparin depends on a number of factors, such as a patient's age, body weight and the amount of time it takes for a sample of the patient's blood to clot. Calculating heparin doses is a complicated process, requiring the use of mathematical formulas and monitoring of the patient.

Calculate the lean body weight. For male patients, subtract 60 from patient's height in inches. Multiply the result times 2.3. Add 50. For female patients, subtract 60 from patient's height in inches. Multiply the result times 2.3. Add 45.

Calculate patient's dosing weight. Consider a patient to be obese if his actual weight in kilograms is more than 1.4 times the lean body weight. For non-obese patients, the dosing weight is the same as the actual weight in kilograms. For obese patients, subtract lean body weight from actual body weight. Multiply the result times 0.4. Add this to the lean body weight.

Give the patient an initial bolus dose of 80 units for every kilogram of dosing weight if he is under 70 years of age. Patients 70 and over should receive 50 units for every kilogram of dosing weight. Be sure to round to the nearest 100 units; do not exceed 10,000 units.

The patient receives a maintenance dose of 18 units per kilogram of dosing weight every hour. Be sure to round to the nearest 40 units per hour.

Obtain the patient's PTT result six hours after initial bolus dose. Every hospital or lab has a specific range for PTT times; many strive for a therapeutic goal of 70 to 100 seconds. Once two consecutive PTT times are within range of the therapeutic goal, consider the patient stable and continue the maintenance dosage as needed. PTT times outside this range will require adjustment; the adjustment doses depend on whether the hospital uses the patient's baseline PTT or the heparin response curve as a guide. Sample adjustment charts are below.