Long COVID: Which Blood Tests Actually Help?

Long COVID — formally called "Post-Acute Sequelae of SARS-CoV-2 infection" (PASC) — refers to symptoms that persist or develop more than 4 weeks after a COVID-19 infection. Estimates of how common it is vary widely (10-30% of people who had COVID, with higher rates after severe infections), but the symptoms themselves are well-characterized: fatigue, brain fog, exercise intolerance, autonomic dysfunction (POTS-like symptoms — dizziness on standing, racing heart), sleep problems, persistent shortness of breath, joint pain, and a host of others.

The hard reality: there's no single blood test that diagnoses long COVID. Diagnosis is clinical — based on symptoms occurring in someone with a documented COVID history. What blood testing CAN do is:

• Rule out treatable conditions that mimic long COVID symptoms. Thyroid dysfunction, B12 deficiency, vitamin D deficiency, anemia, and undiagnosed diabetes all produce overlapping symptoms (fatigue, brain fog, exercise intolerance). Finding and treating these is the most reversible part of the workup.
• Identify post-viral inflammation or autoimmune patterns. COVID-19 can trigger autoimmune conditions (Hashimoto's, lupus, rheumatoid arthritis-like syndromes) and sustained low-grade inflammation. ANA screening + hs-CRP catches these.
• Detect cardiac involvement. Cardiac injury (myocarditis, microvascular damage) is well-documented post-COVID and can cause exercise intolerance and chest symptoms. Lipid + ApoB + hs-CRP plus referral for cardiac imaging if symptoms warrant.

The point isn't to find a "long COVID marker" — there isn't one in routine bloodwork. The point is to find what's contributing to your specific symptoms so you can address those contributors while the broader post-viral picture resolves over time (most long COVID resolves over 6-18 months, but the trajectory is more favorable when the contributing issues are identified and corrected).

Energy & Fatigue Panel ($129.99) — the foundational workup. This bundles seven markers that together cover the most common reversible contributors to post-COVID fatigue and brain fog: TSH + Free T4 (thyroid), Iron + TIBC (iron status), Ferritin (iron stores), Vitamin B12 (nervous system support), Vitamin D (immune and overall function), Hemoglobin A1c (metabolic), CBC with Differential (blood health and immune). One blood draw catches what would otherwise require 7 separate orders.

hs-CRP (High-Sensitivity C-Reactive Protein). Frequently elevated in long COVID — represents the ongoing low-grade inflammation that persists after acute infection has cleared. Optimal hs-CRP is below 1.0 mg/L; long COVID patients commonly run 1-3 mg/L (moderate inflammation) or higher. Trending hs-CRP over time gives a marker for whether the inflammation is resolving.

ANA (Antinuclear Antibody) screen. COVID infection is increasingly recognized as a trigger for new autoimmune conditions in genetically susceptible people. Roughly 5-10% of long COVID patients have positive ANA, suggesting a post-viral autoimmune process. Positive ANA warrants follow-up rheumatology evaluation to determine if it's a transient post-viral finding or the start of a specific autoimmune condition (lupus, mixed connective tissue disease, etc.).

Complete thyroid panel. COVID can trigger thyroid dysfunction in multiple ways: direct viral effect on the thyroid (subacute thyroiditis), post-viral Hashimoto's onset, and autonomic dysregulation affecting thyroid hormone signaling. TSH + Free T4 at minimum; add Free T3, Reverse T3, and TPO antibodies if symptomatic with normal TSH.

Cardiac markers. The Heart Health Panel ($74.99) includes Lipid Panel + ApoB + Lp(a) + hs-CRP + HbA1c. Post-COVID cardiac involvement (microvascular damage, persistent inflammation, autonomic dysfunction) is well-documented; establishing a complete cardiac risk baseline matters for patients with chest symptoms, exercise intolerance, or significant fatigue. If you have specific cardiac symptoms (chest pain, palpitations, severe exercise intolerance), also pursue cardiac imaging — echocardiogram or cardiac MRI as appropriate, ordered through a cardiologist.

If the core workup above is unrevealing and symptoms persist, the following additional tests are worth considering:

Cortisol (AM). COVID can dysregulate the HPA axis (the stress-response system). Chronically low morning cortisol is associated with persistent fatigue and POTS-like symptoms. Drawn 7-9 AM for accurate interpretation. Persistently low results warrant endocrinology referral.

EBV antibodies (VCA IgG, VCA IgM, EBNA-1). Some long COVID patients show reactivation of Epstein-Barr Virus (EBV), the cause of mononucleosis that ~95% of adults have already had as a latent infection. EBV reactivation can contribute to fatigue and immune dysfunction. The pattern of antibodies (high VCA IgG with normal IgM and EBNA-1) is consistent with reactivation rather than acute infection.

D-Dimer. A marker of clotting/clot breakdown. Persistently elevated D-dimer post-COVID is associated with microclotting and may indicate ongoing vascular issues. Elevated D-dimer warrants medical evaluation (typically through a hematologist or cardiologist).

Comprehensive Metabolic Panel (CMP). Captures kidney function, liver function, and electrolyte balance — all of which can be affected by COVID, certain post-COVID treatments, and the deconditioning that accompanies long illness.

Sex hormones. Both men (testosterone) and women (estradiol, progesterone) can experience hormonal disruption post-COVID. Worth investigating if symptoms include significant libido changes, menstrual cycle disruption, or hormonal symptom shifts. Use the Men's or Women's Hormone Panel for comprehensive evaluation.

Sleep evaluation (not a blood test, worth flagging). Post-COVID sleep disturbance is common and significantly contributes to fatigue and cognitive symptoms. If sleep quality is poor (frequent awakening, unrefreshing sleep, snoring or witnessed apneas), pursue a sleep evaluation — often an at-home sleep study — because treating sleep-disordered breathing dramatically improves recovery trajectory.

The trajectory of long COVID: for most people, symptoms gradually improve over 6-18 months. Treatment targets the contributing factors identified through workup — there's no single "cure" for long COVID, but addressing each contributor (correcting thyroid, replacing vitamin D, treating sleep apnea, managing autonomic symptoms, building back exercise tolerance carefully) collectively improves the trajectory. About 70-80% of people meaningfully improve within 12 months; a smaller fraction has persistent symptoms beyond 2 years.

What to do with abnormal results:

• Thyroid abnormal: Endocrinology referral or primary care follow-up. Treatment is straightforward (thyroid medication if hypo; antithyroid management if hyper).
• Vitamin D, B12, or iron deficiency: Supplementation per standard protocols (vitamin D 5,000 IU daily, B12 1,000 mcg sublingual daily, iron 325 mg ferrous sulfate every other day). Retest in 3 months.
• Elevated hs-CRP: Anti-inflammatory diet (Mediterranean-style), addressing sleep, gentle exercise progression, omega-3 supplementation. Retest in 8-12 weeks to track resolution.
• Positive ANA: Rheumatology referral to determine if it's a transient finding or evolving autoimmune condition. Follow-up titer and specific antibody testing as appropriate.
• Cardiac markers abnormal: Cardiology referral; may include echocardiogram or cardiac MRI to evaluate for myocardial involvement.

Where to get treated: Many academic medical centers have established "long COVID clinics" with multidisciplinary teams. The CDC maintains a directory at cdc.gov/coronavirus/2019-ncov/long-term-effects/. If a dedicated long COVID clinic isn't accessible, a primary care physician familiar with the literature, plus targeted referrals (endocrinology, cardiology, rheumatology, sleep medicine), can manage most cases.

What NOT to do:

• Don't push through exercise too aggressively. Post-exertional malaise (PEM) — symptom flare after exertion — is a hallmark of long COVID, especially the ME/CFS-like subset. Aggressive exercise programs can worsen the condition long-term. Pacing and gradual reconditioning (with guidance from a knowledgeable provider) is the evidence-based approach.
• Don't fall for "long COVID cure" supplements or treatments. The space attracts a lot of unproven products. Stick with workup-driven interventions targeting identified deficiencies and conditions.