What is Long COVID?
Long COVID (post-COVID syndrome) is defined as symptoms persisting or developing after acute COVID-19 infection that continue for more than 12 weeks and cannot be explained by an alternative diagnosis. It affects an estimated 10–20% of people after COVID-19 infection, regardless of initial severity. Common symptoms include: extreme fatigue (post-exertional malaise), brain fog, breathlessness, chest pain, palpitations, sleep disturbance, anxiety, and persistent cough.
Blood Tests for Post-COVID Monitoring
| Test | What it Checks | Significance in Long COVID |
|---|---|---|
| D-dimer | Clotting activity / thrombosis risk | Persistently elevated D-dimer >500 ng/mL suggests ongoing microclotting or residual thromboinflammation; >1000 ng/mL warrants imaging to exclude DVT/PE |
| CRP and ESR | Persistent systemic inflammation | Elevated weeks after infection indicates ongoing inflammatory state; should normalise within 4–6 weeks of recovery |
| Ferritin | Iron stores and inflammatory marker | Persistently elevated ferritin (>300 in women, >400 in men) is a marker of ongoing hyperferritinaemia in long COVID inflammatory state |
| CBC (Complete Blood Count) | Immune cell patterns | Lymphopenia (low lymphocytes) and neutrophilia common in acute and some long COVID; anaemia can worsen fatigue |
| TSH (Thyroid) | Post-COVID thyroiditis | Subacute thyroiditis and Graves' disease have been reported post-COVID; can cause fatigue, palpitations, and mood changes |
| Cortisol | Adrenal function | Adrenal involvement reported; low cortisol can cause profound fatigue, low BP, hypoglycaemia |
| Troponin and BNP/NT-proBNP | Cardiac involvement | Post-COVID myocarditis and cardiac dysfunction; elevated troponin in chest pain/palpitation warrants cardiac imaging (echo, cardiac MRI) |
| Vitamin D and B12 | Nutritional status | Deficiency of either worsens fatigue, cognitive symptoms and immune function; commonly deficient and easily treated |
Common Long COVID Symptoms and Corresponding Tests
| Symptom | Key Tests |
|---|---|
| Extreme fatigue / post-exertional malaise | CBC, ferritin, TSH, Vitamin D, B12, cortisol, CRP |
| Brain fog / poor concentration | TSH, Vitamin B12, Vitamin D, glucose, sleep assessment |
| Palpitations / racing heart | ECG, TSH, CBC, troponin, electrolytes |
| Breathlessness | CBC (anaemia), D-dimer, troponin, spirometry (PFT), chest X-ray |
| Chest pain | ECG, troponin, D-dimer, BNP, echocardiogram |
What to Do with Mild Abnormalities
Most mild abnormalities in post-COVID blood tests (slightly elevated CRP, mildly elevated D-dimer 500–1000 ng/mL, mild lymphopenia) resolve with time and do not require specific treatment. Very high D-dimer (>1000 ng/mL) or symptoms of clot (leg swelling, breathlessness) warrants imaging. Persistent severe anaemia, very high troponin, or very elevated ferritin needs specialist review. Focus on gradual recovery — most long COVID improves within 12 months.
Recovery Tips for Long COVID
Pacing is essential — overexertion triggers post-exertional malaise setbacks. Keep an energy diary. Start activity very gradually (e.g. 2–5 minutes walking) and increase by no more than 10–20% per week only if symptom-free. Prioritise sleep and nutrition. Correct any identified deficiencies (Vitamin D, B12, iron). Long COVID clinics offer multidisciplinary support — ask for referral if symptoms persist beyond 3 months.
Questions to Ask Your Doctor
- Which post-COVID blood tests are appropriate for my symptoms?
- My D-dimer is elevated — do I need imaging to exclude a clot?
- Should my thyroid be checked — I have new palpitations and fatigue?
- Should I be referred to a long COVID clinic?
- What activity level is safe for me given my current symptoms?