In an adult with confirmed or suspected COVID-19 and risk factors for severe disease, should we obtain prothrombin time (PT) with international normalized ratio (INR), activated partial thromboplastin time (aPTT), D-dimer, and ferritin on admission?

Laboratory Testing on Admission for COVID-19 Patients with Risk Factors

Yes, obtain PT/INR, aPTT, D-dimer, platelet count, and fibrinogen on admission for all COVID-19 patients with risk factors for severe disease, as these parameters are essential for risk stratification and guiding thromboprophylaxis decisions. 1

Risk Stratification Parameters

The International Society for Thrombosis and Haemostasis (ISTH) specifically recommends obtaining these coagulation markers for risk stratification at presentation 1:

• D-dimer markedly elevated (3-4 fold above normal) indicates need for hospital admission and close monitoring 1
• PT prolonged (report as PT ratio, not INR alone, as INR may miss subtle changes) 1
• Platelet count <100 × 10⁹/L suggests higher risk 1
• Fibrinogen <2.0 g/L warrants closer observation 1

The American Society of Hematology (ASH) similarly recommends monitoring D-dimer, PT, platelet count, and fibrinogen, noting that worsening parameters predict need for aggressive critical care 1.

Specific Thresholds for Action

D-dimer is the single most important prognostic marker 1:

• D-dimer >6 times upper limit of normal is a consistent predictor of thrombotic events and poor prognosis 1
• Markedly elevated D-dimer at admission correlates with mortality 1

Ferritin should also be obtained as it helps identify severe inflammatory response and high thrombotic risk, though it is not specifically a coagulation parameter 2.

Monitoring Strategy After Admission

For Critically Ill Patients

Monitor the following parameters every 24-72 hours during the acute phase (first 7-10 days when thrombotic risk is highest) 1:

• D-dimer every 24-48 hours during first 7-10 days 1
• Platelet count, PT, and fibrinogen every 24-72 hours 1
• Worsening parameters indicate need for escalation of anticoagulation and critical care support 1

For Ward Patients

• Platelet count once or twice weekly to detect heparin-induced thrombocytopenia if using unfractionated heparin 1
• D-dimer monitoring less frequently than ICU patients but still regularly 1

Important Caveats and Pitfalls

Do not use INR alone - PT should be reported as PT ratio, as INR may not detect subtle prolongations that are clinically significant in COVID-19 1. The PT ratio target is <1.5 1.

aPTT has limited utility for monitoring unfractionated heparin in COVID-19 due to hyperinflammatory state 1. Use anti-Xa activity instead (target 0.5-0.7 IU/mL for therapeutic dosing) 1.

Fibrinogen interpretation requires caution - While initially proposed as a thrombotic risk marker, fibrinogen levels have not been consistently associated with thrombotic risk in most studies 1. However, fibrinogen remains useful for diagnosing DIC and heparin-induced thrombocytopenia 1.

Thrombocytopenia is uncommon at admission in COVID-19 compared to bacterial sepsis, but when present (<150 × 10⁹/L), it correlates with disease severity 1.

Clinical Context

Unlike bacterial sepsis-induced coagulopathy, COVID-19 coagulopathy presents with 3, 4:

• Hypercoagulability rather than consumption coagulopathy 4
• Massive D-dimer elevation with relatively normal or elevated fibrinogen 3, 4
• Normal or increased platelet counts (not decreased) 4
• Minimal PT/aPTT prolongation 3

This distinct pattern makes baseline coagulation testing essential for proper risk stratification and anticoagulation management 2, 3.