According to a nationwide population-based study accepted for publication in the journal Scientific Reports, researchers examining persistent post-COVID symptoms found no convincing clinical, neurological, or paraclinical biomarkers to definitively diagnose long COVID. Investigators tracked a cohort of residents in the Faroe Islands who contracted COVID-19 during the early phase of the pandemic, utilizing regular clinical exams, neurological testing, and routine blood markers to search for a measurable biological signature.
How the Faroe Islands Cohort Study Tracked Long COVID Symptoms
In March 2020, the Ministry of Health in the Faroe Islands formed a specialized task force tasked with tracking all PCR-verified cases of COVID-19 and their close contacts every 48 hours to evaluate symptoms and hospitalization requirements. Out of the study data, an aggregate of 324 people infected during either the spring (March–April) or autumn (August–October) of 2020 met eligibility criteria and received invitations to take part, with each having undergone at least 3 months of monitoring. Over an 8-month timeframe, participants filled out baseline and symptom questionnaires through telephone interviews performed up to 6 times, looking back at symptoms felt during the acute phase of infection and whether those problems continued.
The follow-up phase ran from September 2020 to March 2021, capturing comprehensive background information such as smoking habits, medication use, and chronic illnesses. A 21-item symptom questionnaire rated problems on a 0–3 scale. Clinical examinations measured vital signs alongside heart and lung health, while neurological evaluations covered cranial nerves, muscle strength, sensation, reflexes, tactile and vibration sensitivity, and neuromotor control. Additional tests evaluated smell, taste, vision, and hearing, complemented by electrocardiograms (ECGs), lung function tests (spirometry), and blood tests covering hematological, renal, liver, and endocrine markers.
Out of 324 eligible individuals, 273 participated in the clinical follow-up, with 218 included in the final analysis after an average follow-up period of 192 days. According to the findings, 41% reported moderate or severe symptoms, classifying them as having long COVID for the study’s statistical analyses. The study authors cautioned that this relatively high proportion likely did not reflect the true prevalence of the condition in the population. Compared to participants without the condition, those categorized as having long COVID exhibited a greater number of symptoms, an elevated body mass index (BMI), a higher frequency of hospital admissions during the acute stage of illness, and a larger proportion of smokers.
Multivariable analysis showed no prespecified baseline predictors were significantly associated with long COVID, except that having more acute-phase symptoms was borderline significant. Clinical and neurological assessments displayed no meaningful links in univariable analyses, whereas individuals from the first wave demonstrated higher odds of developing long COVID relative to those in the second wave, though investigators noted the underlying cause for this disparity remained unclear. While higher neutrophil counts were associated with long COVID in univariable analysis, that was not confirmed in the adjusted (penalized) model.
Understanding the Lower INR Finding in Long COVID Patients
Blood biomarker analysis identified a statistically significant association between lower mean international normalized ratio values and long COVID. According to the study, participants with long COVID had a lower mean INR of 0.9, compared to 1.0 for those without. INR is a standardized measure of how quickly blood clots and is commonly used to assess coagulation, particularly in people taking anticoagulant medication. However, the authors emphasized that all INR values were within the normal range, meaning no clinical significance was observed and the finding should not be interpreted as evidence of abnormal blood clotting.
Did You Know?
Roughly 6% of people who contract COVID-19 are estimated by current figures to develop long COVID, though this rate fluctuates depending on the population and environment. Long COVID has been linked to upwards of 200 different symptoms, with severe fatigue, cognitive dysfunction, and dysautonomia standing out as some of the most disabling.
Limitations and the Ongoing Search for Biomarkers
The study highlights the ongoing challenge of identifying reliable biomarkers for long COVID. Study investigators pointed out that limitations included a relatively small sample size, reliance on participants self-reporting how severe their symptoms were, the lack of an uninfected control group, conducting multiple exploratory analyses, and an absence of internal or external validation. As a result, the authors pointed out that larger and more rigorously structured studies must validate these findings before any clinical significance can be established.
Frequently Asked Questions
What is long COVID?
Long COVID refers to a condition where a substantial proportion of people continue to experience symptoms for weeks or even months after contracting COVID-19—such as fatigue and loss of smell or taste.
Did the Scientific Reports study find a reliable biomarker?
No. The study did not find convincing biomarkers for long COVID, though it observed a statistically significant lower mean INR value in participants with long COVID.
Are blood clotting abnormalities proven in long COVID patients by this study?
No. While researchers recorded lower mean INR values in long COVID participants, all INR values were within the normal range, so no clinical significance was observed and the finding should not be interpreted as evidence of abnormal blood clotting.
Explore our latest coverage on medical research, public health trends, and clinical studies to stay informed on emerging healthcare developments.
Related reading