What labs are required before starting anticoagulation (blood thinner) therapy?

Required Laboratory Tests Before Starting Anticoagulation Therapy

Before initiating anticoagulation therapy, a baseline complete blood count, renal function tests, and coagulation studies including prothrombin time (PT)/international normalized ratio (INR) and activated partial thromboplastin time (aPTT) should be obtained. 1

Core Laboratory Tests

• Complete blood count (CBC) with platelet count - Essential to rule out thrombocytopenia which would increase bleeding risk 1
• Serum electrolytes and renal function tests - Critical for dosing, especially for direct oral anticoagulants (DOACs) and dabigatran which require dose adjustment for renal impairment 1
• Prothrombin time (PT)/International Normalized Ratio (INR) - Establishes baseline coagulation status and is essential for monitoring vitamin K antagonists like warfarin 1
• Activated partial thromboplastin time (aPTT) - Provides baseline coagulation assessment and is useful for monitoring certain anticoagulants 1

Additional Tests Based on Clinical Context

• Liver function tests - Important for patients with suspected liver disease as hepatic impairment affects drug metabolism and coagulation factor production 1
• Blood glucose - Hyperglycemia is associated with unfavorable outcomes in patients requiring anticoagulation, particularly in stroke patients 1
• ECG - Recommended to identify cardiac conditions that may influence anticoagulation management, particularly for patients with suspected atrial fibrillation 1

Specific Considerations by Anticoagulant Type

For Vitamin K Antagonists (Warfarin)

• Baseline PT/INR is essential for subsequent monitoring 1, 2
• MNPT (Mean Normal Prothrombin Time) should be established for accurate INR calculation 3, 4
• Consider using SAMe-TT2R2 score to identify patients likely to maintain good INR control 1

For Direct Oral Anticoagulants (DOACs)

• Renal function is critical as DOACs have varying degrees of renal clearance:
  • Dabigatran requires more significant dose adjustment for CrCl <50 mL/min 1
  • Apixaban dose reduction is based on age, weight, and creatinine 1
• Liver function tests are important as rivaroxaban and apixaban are partially metabolized by the liver 1

For Heparin/Low Molecular Weight Heparin

• Baseline aPTT is particularly important for monitoring unfractionated heparin 1
• Platelet count monitoring is essential due to risk of heparin-induced thrombocytopenia 1, 5

Important Clinical Considerations

• Laboratory testing should not delay anticoagulation in emergency situations (e.g., stroke) unless there is clinical suspicion of bleeding abnormality, thrombocytopenia, or unknown anticoagulant use 1
• For patients with cancer requiring anticoagulation, baseline laboratory testing should include CBC, renal function, and coagulation studies before initiating therapy 1
• In elderly patients, more frequent monitoring may be required due to increased risk of hemorrhage and fluctuations in the INR 1

Common Pitfalls to Avoid

• Do not delay thrombolytic therapy while waiting for coagulation test results unless there is clinical suspicion of bleeding abnormality, the patient has been taking anticoagulants, or anticoagulant use is uncertain 1
• Do not rely solely on PT/INR for monitoring DOACs as these tests have important limitations for assessing DOAC activity 1
• Do not overlook drug interactions that may affect anticoagulation efficacy and safety 2
• Do not assume normal renal function without testing, especially in elderly patients who may have reduced clearance despite normal serum creatinine 1

By ensuring these laboratory tests are completed before initiating anticoagulation therapy, clinicians can optimize dosing, reduce complications, and establish appropriate baseline values for subsequent monitoring.