POST-COVID RECOVERY
Long COVID Blood Test UK: 10 Biomarkers That Explain Why You Still Don't Feel Right
It's been months since your infection cleared. Maybe longer. The PCR is negative, the acute phase is over, your GP says you've “recovered.” But you haven't. The fatigue is relentless. Your concentration is shot. Your heart races when you climb stairs. You get breathless doing things that used to be effortless. And every time you push through a normal day, you pay for it the next.
Long COVID — formally post-COVID-19 syndrome in NICE terminology — affects an estimated 1.9 million people in the UK. The ONS reports that 72% of those with self-reported long COVID say it limits their daily activities. Yet the standard NHS blood tests ordered for persistent fatigue often come back “normal” — because they're looking for the wrong things.
The problem is not that nothing is wrong. The problem is that standard panels miss the specific patterns of damage that COVID-19 leaves behind — subclinical inflammation, thyroid disruption, metabolic shifts, micronutrient depletion, and HPA axis dysregulation. This guide covers the 10 biomarkers that catch what standard tests miss, what the results mean when read together, and what to do when they confirm that your body is still fighting a battle your GP thinks is over.
1. What long COVID actually is (and why blood tests matter)
NICE guideline NG188 defines post-COVID-19 syndrome as signs and symptoms that develop during or after an infection consistent with COVID-19, continue for more than 12 weeks, and are not explained by an alternative diagnosis. The earlier phase — 4 to 12 weeks — is termed ongoing symptomatic COVID-19.
The most commonly reported symptoms are fatigue (reported by up to 64% of long COVID patients in The Lancet's 2021 Wuhan cohort study), cognitive dysfunction (brain fog), breathlessness, heart palpitations, muscle pain, and sleep disturbance. Many patients report symptoms cycling — improving for days, then crashing after exertion (post-exertional malaise).
What makes blood tests critical is that many of these symptoms have identifiable biochemical drivers. Persistent inflammation shows up in hs-CRP. Thyroid disruption shows up in TSH and free T4. Metabolic damage shows up in HbA1c. Iron depletion shows up in ferritin. These are not guesses — they are measurable, trackable, and in many cases treatable.
The challenge is that a single standard NHS blood test checks only a fraction of what's relevant. A comprehensive panel looking at inflammation, thyroid, metabolic, nutritional, and hormonal markers together reveals patterns that isolated tests miss entirely.
2. The 10 biomarkers that reveal post-COVID damage
These are not the only markers affected by long COVID, but they are the 10 most clinically actionable — the ones where an abnormal result directly informs what to do next.
| Biomarker | What it reveals in long COVID | Typical direction |
|---|---|---|
| hs-CRP | Persistent low-grade systemic inflammation | Elevated (1–10 mg/L) |
| Ferritin | Iron storage depletion OR inflammation-driven elevation | Low or paradoxically high |
| TSH + Free T4 | Post-COVID thyroiditis, subclinical hypothyroidism | TSH elevated, T4 low-normal |
| HbA1c | New-onset insulin resistance or pre-diabetes | Elevated (>39 mmol/mol) |
| Vitamin D | Immune modulation, recovery capacity | Low (<50 nmol/L) |
| Cortisol | HPA axis dysregulation post-infection | Blunted or elevated |
| Vitamin B12 | Neurological symptoms, cognitive dysfunction | Low-normal or deficient |
| ALT | Post-COVID liver inflammation | Mildly elevated |
| Full blood count | Lymphopenia, immune cell imbalance | Low lymphocytes |
| Iron + transferrin sat. | Functional iron deficiency despite “normal” ferritin | Low transferrin saturation |
The critical insight is that long COVID rarely shows a single wildly abnormal result. It shows a pattern — multiple markers slightly shifted in the same direction. Reading them in isolation says “normal.” Reading them together says “your body is still inflamed, depleted, and dysregulated.”
3. hs-CRP: the inflammation that won't resolve
High-sensitivity C-reactive protein is the single most important marker for understanding persistent post-COVID inflammation. CRP is produced by the liver in response to inflammatory cytokines — IL-6 in particular — and in acute COVID-19 infection, CRP levels can spike above 100 mg/L.
In healthy recovery, CRP returns to baseline within 2–4 weeks after infection resolves. In long COVID, it often doesn't. A 2022 Nature Medicine study of over 150,000 individuals found that people with long COVID symptoms had persistently elevated inflammatory markers compared to matched controls, even 12 months after infection.
The NHS standard CRP test has a detection limit of around 5 mg/L — designed to catch major infections and inflammatory conditions. The high-sensitivity version (hs-CRP) detects levels as low as 0.1 mg/L, which is exactly the range where long COVID inflammation typically sits: 1–8 mg/L. Clinically “normal” by standard criteria, but significantly above the optimal level of below 1.0 mg/L.
If your hs-CRP remains above 3 mg/L more than 12 weeks after COVID, that is objective evidence that systemic inflammation has not resolved — regardless of how your GP interprets the result.
4. Ferritin and iron studies: fatigue's hidden driver
Ferritin is an acute-phase reactant — meaning it rises during inflammation regardless of iron status. During acute COVID-19, ferritin can reach 1,000–10,000 µg/L as part of the cytokine storm. This creates a diagnostic trap in long COVID: ferritin may look “normal” or even elevated, while the body's actual usable iron is depleted.
This phenomenon — functional iron deficiency with normal or raised ferritin — is well-documented in chronic inflammatory conditions. The NICE guideline NG24 on blood transfusion notes that ferritin alone is unreliable in the presence of inflammation.
The solution is to look at the full iron panel: serum iron, transferrin saturation, and total iron-binding capacity (TIBC) alongside ferritin and hs-CRP. In long COVID, the classic pattern is:
- Ferritin normal or raised (driven by inflammation, not iron stores)
- Transferrin saturation low (<20%) — indicating inadequate iron delivery to tissues
- hs-CRP mildly elevated — confirming the inflammatory context
- Haemoglobin borderline — not low enough for an anaemia diagnosis, but low enough to drive fatigue
If your GP tests ferritin alone and says “your iron is fine,” but your transferrin saturation is 15% and your hs-CRP is 4 mg/L, your iron is not fine. Your body is sequestering iron as part of an ongoing inflammatory response.
5. Thyroid function: COVID's silent endocrine target
COVID-19 has a documented affinity for thyroid tissue. The SARS-CoV-2 virus binds to ACE2 receptors, which are expressed in the thyroid gland. A Lancet Diabetes & Endocrinology review documented subacute thyroiditis, Graves' disease onset, and destructive thyroiditis following COVID-19 infection.
The most common post-COVID thyroid pattern is subclinical hypothyroidism — where TSH creeps above 4 mIU/L but stays below 10 mIU/L (the threshold where most GPs intervene), while free T4 sits at the lower end of normal. NICE clinical knowledge summary on hypothyroidism notes that subclinical hypothyroidism may resolve spontaneously — but in post-COVID patients, the fatigue, cold intolerance, brain fog, and weight gain it causes are often attributed to long COVID itself, and the thyroid is never checked.
Some patients experience the opposite: a transient thyrotoxic phase (low TSH, high free T4) from destructive thyroiditis, which then swings into hypothyroidism as the gland recovers. Without testing at the right time, both phases can be missed entirely.
A minimum thyroid panel should include TSH and free T4. Ideally, free T3 as well — because some post-COVID patients show impaired T4-to-T3 conversion (known as low T3 syndrome or euthyroid sick syndrome), where TSH and T4 look acceptable but the active thyroid hormone is deficient.
6. HbA1c: the metabolic damage COVID leaves behind
One of the more alarming findings from post-COVID research is the increased incidence of new-onset diabetes. A Lancet Diabetes & Endocrinology analysis of over 180,000 US veterans found that COVID-19 survivors had a 40% increased risk of developing diabetes in the 12 months after infection, compared to contemporary controls.
The mechanism is believed to be direct viral damage to pancreatic beta cells (which produce insulin) combined with inflammation-driven insulin resistance. Even in patients who don't develop overt diabetes, COVID can push HbA1c from a healthy range into the pre-diabetic zone (42–47 mmol/mol) — where energy crashes, brain fog, and weight gain become daily features.
If you had a normal HbA1c before COVID and now experience unexplained fatigue, sugar cravings, afternoon energy crashes, or difficulty losing weight, rechecking HbA1c is essential. The shift from 37 to 43 mmol/mol is enough to drive noticeable symptoms — and it's entirely reversible at this stage with dietary and activity changes.
7. Vitamin D: immune recovery and beyond
The relationship between vitamin D and COVID-19 outcomes was one of the earliest and most studied associations of the pandemic. A BMJ meta-analysis and subsequent studies found that vitamin D deficiency was associated with increased COVID-19 severity, ICU admission, and mortality.
For long COVID specifically, vitamin D matters for two reasons. First, it modulates the immune response — specifically the balance between inflammatory Th1/Th17 cells and regulatory T cells. Persistent immune dysregulation is a hallmark of long COVID, and adequate vitamin D supports the shift from inflammation to resolution.
Second, vitamin D deficiency is endemic in the UK. The SACN report on vitamin D found that approximately 1 in 5 UK adults have serum levels below 25 nmol/L (deficiency), with higher rates in winter months, in older adults, and in those with darker skin.
The NHS defines sufficiency as >25 nmol/L, but most evidence-based practitioners target 75–125 nmol/L for optimal immune function. If you're recovering from long COVID and your vitamin D is 30 nmol/L, you're technically “adequate” by NHS standards but significantly below the threshold associated with better outcomes.
8. Cortisol and HPA axis dysfunction
Research published in Nature Medicine identified lower morning cortisol as one of the strongest biological predictors of long COVID. The study found that patients with long COVID had significantly reduced morning cortisol compared to those who recovered fully.
This pattern — blunted cortisol output from a dysregulated hypothalamic-pituitary-adrenal axis — mirrors what happens in other post-infectious fatigue syndromes, including ME/CFS. The body's stress response system, after weeks of fighting a systemic infection, fails to recalibrate properly.
Low morning cortisol explains several hallmark long COVID symptoms: unrefreshing sleep (cortisol should spike on waking to provide alertness), exercise intolerance (cortisol is needed to mobilise glucose during physical activity), and the “crash” pattern where normal activities lead to disproportionate exhaustion.
Testing must be done as a morning sample — ideally between 8:00 and 10:00 am, when cortisol should be at its peak. An afternoon test is meaningless for this assessment. If morning cortisol is below 300 nmol/L with symptoms consistent with long COVID, further investigation including DHEA-S (the cortisol:DHEA-S ratio) is warranted.
9. Vitamin B12: the neurological connection
Brain fog, tingling in extremities, difficulty concentrating, and memory problems are among the most debilitating long COVID symptoms. While multiple mechanisms contribute — neuroinflammation, microvascular damage, autoimmunity — vitamin B12 deficiency produces almost identical symptoms and is easily treatable.
B12 is essential for myelin synthesis (the insulating sheath around nerve fibres) and for the production of neurotransmitters including serotonin and dopamine. The NICE guideline CG151 on anaemia management notes that B12 deficiency can cause irreversible neurological damage if left untreated.
COVID-19 may deplete B12 through several pathways: increased metabolic demand during acute infection, gastrointestinal disruption affecting absorption (GI symptoms are common in COVID), and medication effects (proton pump inhibitors, metformin, and some antibiotics used during COVID treatment all reduce B12 absorption).
The NHS defines B12 deficiency as below 148 pmol/L. But neurological symptoms can manifest at levels that are technically “normal” — particularly between 148 and 300 pmol/L. If you have post-COVID neurological symptoms and your B12 is 180 pmol/L, supplementation is worth discussing with your GP despite the “normal” result.
10. Liver function and full blood count
COVID-19 can cause direct hepatic injury. Studies published in The BMJ reported elevated liver enzymes in 14–53% of hospitalised COVID patients. While most cases resolve, some patients show persistently elevated ALT months after infection — suggesting ongoing hepatic inflammation.
ALT (alanine aminotransferase) is the most specific marker for liver cell damage. Persistently elevated ALT (>35 U/L in women, >45 U/L in men) warrants investigation, particularly in the context of long COVID where multiple organ systems may be affected. NICE guideline NG49 on non-alcoholic fatty liver disease provides the investigation pathway.
The full blood count (FBC) reveals immune cell patterns that persist after COVID. The most common finding is lymphopenia — a reduced lymphocyte count that reflects ongoing immune system activation or exhaustion. A BMJ rapid review noted that lymphopenia can persist for months after acute infection and correlates with symptom burden.
An elevated monocyte count or raised neutrophil-to-lymphocyte ratio (NLR) further supports the picture of ongoing immune activation. None of these findings are diagnostic alone, but in the context of persistent symptoms, they add objective weight to what patients are experiencing.
11. NHS ranges vs optimal recovery ranges
The NHS reference ranges are designed to exclude pathology — to identify disease. They are not designed to identify suboptimal function. In long COVID recovery, the gap between “not diseased” and “actually well” is where most patients live.
| Biomarker | NHS “normal” | Optimal recovery target |
|---|---|---|
| hs-CRP | <5 mg/L | <1.0 mg/L |
| Ferritin | 15–300 µg/L (men) | 40–150 µg/L (with normal CRP) |
| TSH | 0.4–4.0 mIU/L | 0.5–2.5 mIU/L |
| HbA1c | <42 mmol/mol | <36 mmol/mol |
| Vitamin D | >25 nmol/L | 75–125 nmol/L |
| Cortisol (morning) | 166–507 nmol/L | 350–500 nmol/L |
| Vitamin B12 | >148 pmol/L | >300 pmol/L |
| ALT | <35–45 U/L | <25 U/L |
| Transferrin sat. | 15–50% | 25–45% |
| Lymphocytes | 1.0–4.0 ×10⁹/L | >1.5 ×10⁹/L |
The crucial point: a result that falls within the NHS reference range does not mean it is compatible with feeling well. A TSH of 3.8 mIU/L, an hs-CRP of 4.5 mg/L, a vitamin D of 28 nmol/L, and a B12 of 160 pmol/L are all “normal” — and together they describe someone who feels terrible. In post-COVID recovery, optimal ranges matter more than reference ranges.
12. Five long COVID blood patterns and what to do about each
Long COVID rarely shows one isolated abnormality. It shows constellations. Here are the five most common patterns we see.
Pattern 1: The smouldering inflammation
Markers: hs-CRP 2–8 mg/L, ferritin elevated relative to iron, lymphocytes low-normal, ALT mildly raised.
Symptoms: Persistent fatigue, muscle aches, brain fog that worsens with exertion, intermittent low-grade fever.
Direction: Discuss anti-inflammatory strategies with your GP. Evidence-based interventions include omega-3 supplementation (EPA/DHA 2–3g/day per NICE CG181), graded activity within post-exertional malaise limits, and optimising vitamin D. Retest at 8 weeks.
Pattern 2: The thyroid disruption
Markers: TSH 4–10 mIU/L, free T4 low-normal, hs-CRP mildly elevated.
Symptoms: Cold intolerance, weight gain, dry skin, constipation, fatigue worse in mornings, brain fog.
Direction: Retest thyroid at 6–8 weeks. If TSH remains >4 mIU/L with symptoms, discuss levothyroxine trial with GP per NICE CKS on hypothyroidism. Check thyroid antibodies (anti-TPO) to differentiate autoimmune thyroiditis from viral thyroiditis.
Pattern 3: The metabolic shift
Markers: HbA1c 39–47 mmol/mol (up from pre-COVID baseline), fasting glucose borderline, hs-CRP mildly elevated.
Symptoms: Energy crashes after meals, sugar cravings, unexplained weight gain, difficulty concentrating after eating.
Direction: This is the most reversible pattern. Dietary intervention (reducing refined carbohydrate, increasing fibre and protein at every meal), regular movement, and improved sleep can bring HbA1c back to baseline within 3–6 months. The NICE NG28 diabetes prevention pathway applies.
Pattern 4: The nutritional depletion
Markers: Vitamin D <50 nmol/L, B12 <300 pmol/L, ferritin <30 µg/L (or transferrin sat. <20%), magnesium low-normal.
Symptoms: Fatigue, hair loss, muscle weakness, tingling, mood changes, poor wound healing.
Direction: Targeted supplementation based on confirmed deficiencies. Vitamin D3 loading dose per SACN guidance, B12 injections if <148 pmol/L or sublingual/oral if borderline, iron supplementation with vitamin C for absorption. Retest at 12 weeks to confirm repletion.
Pattern 5: The HPA axis crash
Markers: Morning cortisol <300 nmol/L, DHEA-S low for age, vitamin D low, thyroid borderline.
Symptoms: Severe exercise intolerance, post-exertional malaise (crash after normal activity), unrefreshing sleep, orthostatic intolerance (dizziness on standing).
Direction: This is the pattern that most closely mirrors ME/CFS. Activity pacing is essential — pushing through makes it worse. Discuss with your GP. The NICE guideline NG206 on ME/CFS provides a management framework that applies to this post-COVID pattern. Support adrenal recovery with adequate sleep, stress reduction, and targeted supplementation (vitamin D, magnesium, B vitamins).
13. Which Helvy panel covers what
No single panel covers all 10 biomarkers — because long COVID affects multiple systems. Here's how the Helvy panels map to the markers discussed in this guide.
| Biomarker | Essential (£89) | Performance (£149) | Heart (£129) |
|---|---|---|---|
| hs-CRP | — | ✓ | ✓ |
| Ferritin | ✓ | ✓ | — |
| TSH | ✓ | ✓ | — |
| HbA1c | ✓ | ✓ | ✓ |
| Vitamin D | ✓ | ✓ | — |
| Cortisol | — | ✓ | — |
| Vitamin B12 | ✓ | ✓ | — |
| ALT (liver) | ✓ | ✓ | — |
| Full blood count | ✓ | ✓ | — |
| Iron studies | ✓ | ✓ | — |
For comprehensive post-COVID assessment, the Performance panel (£149) provides the broadest coverage — including cortisol and DHEA-S, which are critical for identifying HPA axis dysfunction and are not included in the Essential panel.
If cardiovascular symptoms (palpitations, chest tightness, exercise intolerance) are prominent, adding the Heart panel (£129) provides hs-CRP plus ApoB and Lp(a) to assess whether COVID has affected your cardiovascular risk profile.
14. When and how to test after COVID
| Timing | What to test | Why |
|---|---|---|
| 8–12 weeks post-infection | Full panel: hs-CRP, ferritin, iron studies, TSH, FT4, HbA1c, vitamin D, B12, FBC, ALT, cortisol | Baseline to assess post-COVID damage vs pre-existing conditions |
| 6 months post-infection | Retest any abnormal markers plus HbA1c | Track resolution vs persistence; HbA1c reflects the 3–6 month window |
| 12 months post-infection | Full retest if symptoms persist | Long-term assessment; distinguish residual post-COVID effects from new developments |
Testing protocol
- Fasting sample: 10–12 hours overnight fast for accurate HbA1c and iron studies. Water is fine. See our fasting blood test guide for the full protocol.
- Morning sample: Before 10:00 am for cortisol (peaks on waking, then declines throughout the day).
- Avoid intense exercise 24 hours before: Exercise transiently raises CRP, cortisol, and liver enzymes, which could confound your results.
- Note your symptoms that week: Were you in a “crash” phase or a “good” phase? This context helps interpret borderline results.
GP vs private testing for long COVID
Most GPs will order FBC, CRP, thyroid, ferritin, and HbA1c if you report persistent post-COVID symptoms. This covers roughly half the panel above. What's typically missing from GP-ordered tests:
- hs-CRP (GPs usually get standard CRP, which misses the low-grade inflammation range)
- Full iron studies (GPs often test ferritin alone, missing functional iron deficiency)
- Morning cortisol (rarely ordered unless Addison's or Cushing's is suspected)
- Vitamin D (not routinely offered; NICE recommends assuming deficiency and supplementing rather than testing)
- DHEA-S (almost never included in NHS primary care panels)
15. Frequently asked questions
01Can a blood test diagnose long COVID?
There is no single diagnostic blood test for long COVID. The diagnosis is clinical — based on persistent symptoms after confirmed or suspected COVID-19 per NICE NG188. However, blood tests identify the specific biochemical abnormalities driving symptoms (inflammation, thyroid dysfunction, metabolic damage, nutritional depletion) — which is more useful than a label, because it tells you what to treat.
02My GP says my blood tests are normal but I still feel terrible. What should I do?
“Normal” means within the NHS reference range — it does not mean optimal. Ask your GP which specific tests were run (many patients assume “blood tests” covers everything, when it may be just FBC and CRP). Request a copy of the actual results with numbers, not just “normal / abnormal.” Then compare against the optimal recovery ranges in section 11 of this guide.
03How long after COVID should I wait to get tested?
Wait at least 8 weeks after your acute infection resolves. Testing earlier can show residual acute-phase changes (very high CRP, low lymphocytes) that would resolve on their own. The 8–12 week mark gives a clearer picture of what has persisted vs what was transient.
04Does vaccination change my blood test results?
Vaccination can transiently raise CRP and affect lymphocyte subsets for 1–2 weeks. If you've had a recent booster, wait at least 2 weeks before testing for the most accurate inflammatory marker results. Vaccination does not affect HbA1c, thyroid, ferritin, vitamin D, or B12 results.
05Is long COVID the same as ME/CFS?
They are not identical but share significant overlap. Both involve post-exertional malaise, fatigue, cognitive dysfunction, and autonomic symptoms. Some researchers believe long COVID triggers ME/CFS in susceptible individuals. The NICE NG206 guideline on ME/CFS applies to post-COVID patients who meet the diagnostic criteria. The blood test patterns (particularly the HPA axis pattern) are often similar.
06Can I get these tests on the NHS?
Your GP can order FBC, CRP, thyroid function, ferritin, HbA1c, B12, and liver function — which covers approximately 70% of the panel. What you typically won't get on the NHS: hs-CRP (high-sensitivity version), morning cortisol, DHEA-S, full iron studies (transferrin saturation), and vitamin D (NICE recommends supplementing without testing). A private panel fills these gaps.
07Will my long COVID blood markers eventually normalise?
Most do. The Lancet's 2-year follow-up of the Wuhan cohort showed significant improvement in most biomarkers by 24 months. However, a subset of patients (estimated 15–20%) show persistent abnormalities beyond 2 years. Serial testing every 3–6 months tracks your trajectory and confirms whether interventions are working.