What are the guidelines for initiating and monitoring heparin (unfractionated heparin) therapy, including dosing and laboratory tests such as activated Partial Thromboplastin Time (aPTT)?

Heparin Initiation and Monitoring Guidelines

Initial Dosing

For unfractionated heparin (UFH) therapy, initiate with a weight-based bolus of 80 units/kg followed by a continuous infusion of 18 units/kg/hour, as this regimen achieves therapeutic anticoagulation more rapidly and safely than fixed-dose or lower weight-based regimens. 1

Weight-Based Dosing Protocol

• Bolus dose: 80 units/kg IV (maximum 4000 units for patients >70 kg when used with fibrinolytics) 1
• Initial infusion: 18 units/kg/hour (maximum 1000 units/hour for patients >70 kg when used with fibrinolytics) 1
• This higher-dose nomogram (80:18) achieves therapeutic aPTT significantly faster than the older 60:12 regimen, with 97% of patients reaching therapeutic range within 24 hours versus 77% with standard dosing 2
• The 80:18 regimen reduces recurrent thromboembolism risk by 5-fold compared to lower-dose protocols 2

Special Populations

• Pediatric patients: 75-100 units/kg IV bolus over 10 minutes, followed by 25-30 units/kg/hour for infants (highest requirements in those <2 months) or 18-20 units/kg/hour for children >1 year 3
• Cardiovascular surgery: Minimum 150 units/kg, with 300 units/kg for procedures <60 minutes or 400 units/kg for procedures >60 minutes 3
• Obese patients: Do not cap doses based on weight—underdosing is common and delays therapeutic anticoagulation, with each 1 unit/kg/hour reduction causing 0.75-1.5 hour delay in achieving therapeutic range 4

Laboratory Monitoring with aPTT

Target Range and Timing

The therapeutic aPTT target is 1.5-2.3 times control (approximately 46-70 seconds or 60-85 seconds depending on institutional calibration), corresponding to anti-Xa levels of 0.3-0.7 units/mL. 1, 5, 6

• First aPTT measurement: Draw 4-6 hours after initiating heparin infusion (or 6 hours after bolus for continuous infusion) 1, 5, 3
• Subsequent monitoring: Every 4-6 hours initially until therapeutic, then daily once stable 3
• For intermittent IV dosing: Check aPTT before each injection 3
• For subcutaneous dosing: Check 4-6 hours after injection 3

Critical Laboratory Considerations

Each institution must establish its own therapeutic aPTT range calibrated to their specific reagent and coagulometer, as the same heparin concentration can produce aPTT values ranging from 48 to 108 seconds depending on the reagent used. 5

• Different aPTT reagents have vastly different sensitivities to heparin—no universal nomogram applies 5, 7
• The therapeutic range must correspond to anti-Xa levels of 0.3-0.7 units/mL when validated 5, 6
• Subtherapeutic aPTT (<50 seconds) increases recurrent VTE risk 15-fold 5, 6, 7

Dose Adjustment Algorithm

Adjust heparin infusion rates according to the following aPTT-based nomogram: 1, 6

aPTT Result	Action
<35 seconds (<1.2× control)	Give 80 units/kg bolus; increase infusion by 4 units/kg/hour [1,6]
35-45 seconds (1.2-1.5× control)	Give 40 units/kg bolus; increase infusion by 2 units/kg/hour [1,6]
46-70 seconds (1.5-2.3× control)	No change—therapeutic range [1,6]
71-90 seconds (2.3-3.0× control)	Decrease infusion by 2 units/kg/hour [1,6]
>90 seconds (>3.0× control)	Stop infusion for 1 hour, then decrease by 3 units/kg/hour [1,6]

Additional Monitoring Requirements

Platelet Count Surveillance

• Monitor platelet counts daily throughout heparin therapy regardless of route 3
• Screen for heparin-induced thrombocytopenia (HIT), which occurs 10 times less frequently with LMWH than UFH 8

Other Laboratory Tests

• Monitor hematocrit and test for occult blood in stool periodically 3
• Baseline coagulation studies (aPTT, INR, platelet count) before initiating therapy 3

Critical Pitfalls to Avoid

Dosing Errors

• Never use fixed doses—weight-based dosing is essential for achieving rapid therapeutic anticoagulation 1, 2
• Do not underdose obese patients—89% receive inadequate bolus doses and 76% receive inadequate infusions, leading to delayed anticoagulation 4
• Avoid assuming 1.5-2.5× control is therapeutic without validating this range against anti-Xa levels for your specific laboratory 5, 7

Monitoring Failures

• Do not ignore subtherapeutic aPTT values (50-59 seconds)—these still carry significantly increased thrombotic risk 5, 6
• Recognize heparin resistance—consider switching to anti-Xa monitoring (target 0.35-0.7 units/mL) if therapeutic aPTT cannot be achieved 6
• Excessive anticoagulation (aPTT >90 seconds) increases bleeding risk without additional benefit 6, 7

Clinical Context

• Start heparin empirically in patients with suspected PE while awaiting diagnostic confirmation, given high mortality in untreated patients 1
• Always overlap with warfarin for at least 5-7 days until INR is therapeutic (2.0-3.0) for 2 consecutive days 7
• Modify dosing when combining heparin with thrombolytics or GP IIb/IIIa inhibitors (use 60-70 units/kg bolus, maximum 5000 units, then 12-15 units/kg/hour, maximum 1000 units/hour) 1, 6, 7

Alternative to aPTT Monitoring

• Anti-Xa assay: Consider in patients with heparin resistance, lupus anticoagulant, or when aPTT results are unreliable 5, 8
• Target range: 0.3-0.7 units/mL for therapeutic anticoagulation 1, 5, 8
• Draw 4 hours after morning injection for LMWH (0.6-1.0 units/mL for twice-daily dosing, 1.0-2.0 units/mL for once-daily) 1