CureusJun 23, 2026

Blood Clotting Problems and Ongoing Symptoms in Long COVID: What Primary Care Should Know

Updated Jun 24, 2026

Abstract

Elevated D-dimer and fibrinogen levels were observed in 25.4% and 21.8% of 197 adults with long COVID, respectively.

Reduced effort tolerance was reported by 76.7% of participants, making it the most common symptom.
Higher levels of D-dimer and fibrinogen were significantly associated with severe symptoms (p < 0.001 for both).
Patients with elevated D-dimer had a relative risk of 3.0 for severe symptoms, while those with elevated fibrinogen had a relative risk of 2.34.
At three months, 50.8% of patients with initially elevated D-dimer and 47.6% with elevated fibrinogen showed persistent elevations.
Basic coagulation markers and the Six-Minute Walk Test may help assess symptom severity and functional limitations in long COVID patients.

Simplified

Background The long-term sequelae of COVID-19, collectively termed long COVID, manifest as persistent symptoms that may be driven by ongoing coagulation abnormalities. Early identification, particularly in primary care, requires simple, low-cost tools that do not rely on advanced diagnostics. This study evaluated the usefulness of basic coagulation markers and the Six-Minute Walk Test (6MWT) in assessing symptom severity and functional limitation among individuals with long COVID. Methodology A two-phase observational study was conducted from July 2022 to January 2024. Phase I enrolled 197 adults more than six weeks post-COVID-19 who presented with fatigue, breathlessness, reduced exercise tolerance, cough, or musculoskeletal pain. Baseline investigations included C-reactive protein (CRP), prothrombin time (PT)/international normalized ratio (INR), activated partial thromboplastin time (aPTT), D-dimer, fibrinogen, and platelet count, alongside the 6MWT. The primary objective was to evaluate the association between coagulation abnormalities and the severity of long COVID symptoms. Secondary objectives included assessment of functional impairment using the 6MWT and evaluation of persistence of coagulation abnormalities at the three-month follow-up. Patients with abnormal baseline coagulation or inflammatory parameters (n = 63) underwent repeat evaluation after three months. Results The study included 197 participants with a mean age of 37.3 ± 11.97 years, of whom 87 (44.2%) were male. Reduced effort tolerance was the most common presenting symptom, observed in 151 (76.7%) patients. At baseline, abnormalities were noted in D-dimer (50 (25.4%)), fibrinogen (43 (21.8%)), CRP (34 (17.3%)), aPTT (23 (11.7%)), PT/INR (21 (10.7%)), and platelet count (10 (5.1%)). Patients with severe symptoms demonstrated significantly higher levels of D-dimer and fibrinogen (p < 0.001 for both). Elevated D-dimer (50 (25.4%)) and fibrinogen (43 (21.8%)) were associated with an increased risk of severe symptomatology, with relative risks of 3.0 and 2.34, respectively. At the three-month follow-up, persistent elevation of D-dimer (32 (50.8%)) and fibrinogen (30 (47.6%)) was observed, indicating sustained coagulation abnormalities in a substantial subset of individuals with long COVID. Conclusions This study demonstrates a significant association between elevated D-dimer and fibrinogen levels and symptom severity in patients with long COVID. Simple, accessible tools, particularly basic coagulation tests and the 6MWT, may serve as useful adjunctive assessments for primary care physicians to identify long COVID patients with probable ongoing thrombo-inflammatory activity. Integrating these assessments into routine follow-up may improve early detection, monitoring, and targeted referral.