What laboratory tests are used to assess coagulation in a patient?

Laboratory Tests to Assess Coagulation

The essential laboratory tests to check coagulation include prothrombin time (PT)/INR, activated partial thromboplastin time (aPTT), fibrinogen level, and platelet count, with viscoelastic testing (TEG/ROTEM) providing additional real-time assessment of clot formation and strength. 1, 2

Core First-Line Coagulation Tests

Standard Coagulation Panel

• Prothrombin Time (PT)/International Normalized Ratio (INR) evaluates the extrinsic and common coagulation pathways, monitoring factors II, V, VII, and X 1, 2
• Activated Partial Thromboplastin Time (aPTT) assesses the intrinsic and common pathways, evaluating factors VIII, IX, XI, XII, prekallikrein, and high-molecular-weight kininogen 1, 2
• Fibrinogen level (Clauss method preferred) is critical because fibrinogen is often the first coagulation factor to reach critically low levels during major bleeding 1, 2
• Platelet count is essential for assessing primary hemostasis and clot formation capacity 1, 2

Important Limitations of Standard Tests

• PT and aPTT monitor only the initiation phase of blood coagulation, representing just the first 4% of thrombin production 1
• These conventional tests can appear normal while the overall coagulation state remains abnormal 1
• In liver disease patients, traditional coagulation tests are particularly inadequate and misleading because they neglect counterbalanced hemostatic factors 1

Viscoelastic Testing (Point-of-Care)

Thromboelastography (TEG) or Rotational Thromboelastometry (ROTEM)

• Provides comprehensive real-time assessment of clot initiation, formation, strength, and breakdown 1, 2
• Turnaround time is 30-60 minutes faster than conventional laboratory testing 1
• Predicts need for massive transfusion and mortality better than standard tests in trauma and surgical patients 1
• Should be performed to assist in characterizing coagulopathy and guiding hemostatic therapy 1

Clinical Applications

• More valuable than standard tests in liver disease for assessing actual hemostatic capacity 1, 3
• Useful for detecting coagulation abnormalities from direct thrombin inhibitors (dabigatran, argatroban, bivalirudin) 1
• May identify patients who do not need preprocedural correction despite abnormal conventional tests 1

Additional Tests Based on Clinical Context

When Bleeding is Present or Suspected

• Complete blood count (CBC) to assess for anemia from blood loss 2
• Base deficit and lactate levels for assessing shock severity and resuscitation adequacy in trauma patients 1
• D-dimer if disseminated intravascular coagulation (DIC) is suspected 1, 2

For Patients on Anticoagulants

• Specialized assays for direct oral anticoagulants (DOACs): dilute thrombin time or ecarin assays for dabigatran; anti-factor Xa assays for rivaroxaban, apixaban, edoxaban 1
• Thrombin time (TT) can rapidly exclude clinically significant dabigatran levels if normal 1
• PT and aPTT are variably affected by DOACs and generally unhelpful for monitoring their concentrations 4

For Platelet Dysfunction

• Platelet function testing (light transmission aggregometry, PFA-100/200) if patient is on antiplatelet medications or has suspected platelet dysfunction 2, 5
• Von Willebrand factor (VWF) antigen and function testing for suspected von Willebrand disease 5

Monitoring Frequency and Timing

During Active Bleeding or Resuscitation

• Perform early and repeated measurements of PT, aPTT, fibrinogen, and platelets 1
• For continuous IV heparin: check aPTT at baseline, approximately every 4 hours initially, then at appropriate intervals 6
• For intermittent IV heparin: perform coagulation tests before each injection during initiation 6
• Regular monitoring of platelet count, PT, D-dimer, and fibrinogen is important for diagnosing worsening coagulopathy 1

Subcutaneous Heparin

• Tests for adequacy are best performed on samples drawn 4-6 hours after injection 6

Critical Pitfalls to Avoid

• Do not rely solely on PT and aPTT as they may not correlate with clinical bleeding risk 1, 2
• Do not delay treatment while waiting for laboratory results in cases of severe bleeding 2, 3
• A normal PT or aPTT does NOT exclude the presence of direct oral anticoagulants 4
• Traditional tests were not designed to monitor coagulation deficiencies during active hemorrhage 2
• Visual assessment of bleeding remains essential and should be conducted jointly by the clinical team 2
• Periodically monitor platelet counts, hematocrit, and occult blood in stool during the entire course of heparin therapy 6