Prolonged symptoms after Covid-19 (long Covid) in adults

General symptoms · Symptom or situation · updated 2026-10-30

BioMG follows French guidelines. Adapt it to the guidelines and the tests available where you practise.

Frequency: 4% of adults in metropolitan France in April 2022 (about 2 million people), according to the WHO definition ^[1]

Primary care tests (symptoms beyond 4 weeks)

Fasting sample.

Fasting laboratory tests

FBC
Serum electrolytes
Serum creatinine with eGFR
Liver function tests (AST, ALT, GGT, alkaline phosphatase)
CRP
Serum ferritin
Fasting plasma glucose
TSH (if symptoms suggesting thyroid dysfunction)if neededThe HAS rapid response asks for TSH only if the symptoms suggest thyroid dysfunction ^[2]; the HAS “Fatigue” page, updated in 2023, includes it in the standard tests for asthenia (next tab) ^[3].
SARS-CoV-2 serology (only if the initial episode was not confirmed by a test)if neededThe HAS proposes it only in the absence of initial laboratory confirmation ^[2]; the NABM moreover limits the procedure to late aetiological diagnosis in a patient without signs of severity who has not had an RT-PCR ^[4].
Vitamin B12if needednon-French guidelinePersistent cognitive disorders: American Family Physician includes vitamin B12 and folate in the initial tests for brain fog, alongside other assays, to look for a treatable cause ^[5] (expert opinion). The 2024 HAS page on cognitive disorders rests on cognitive tests, neuropsychological assessment and MRI, and does not cite these assays, without advising against them ^[6].
Folateif needednon-French guidelinePersistent cognitive disorders: American Family Physician includes vitamin B12 and folate in the initial tests for brain fog, alongside other assays, to look for a treatable cause ^[5] (expert opinion). The 2024 HAS page on cognitive disorders rests on cognitive tests, neuropsychological assessment and MRI, and does not cite these assays, without advising against them ^[6].

Detailed explanation and international comparison (in French): read it in English (machine translation)

Do not order

SARS-CoV-2 serology when the infection was confirmed by RT-PCR or antigen test: the HAS proposes it only in the absence of initial laboratory confirmation ^[2], and the NABM reserves it for patients who have not had an RT-PCR ^[4].
Extended tests “looking for long Covid”: the HAS asks to avoid escalating irrelevant further tests; beyond the baseline tests, tests are guided by symptoms ^[2,7].

Fatigue as the main symptom

Fasting sample.

Standard tests for asthenia, fasting, in the morning

FBC
Fasting plasma glucose
Serum electrolytes
Serum calcium and serum albumin (corrected calcium)
Serum creatinine with eGFR
Liver function tests (AST, ALT, GGT, alkaline phosphatase)
Serum ferritin
CRP
TSH
8 am serum cortisol

Detailed explanation and international comparison (in French): read it in English (machine translation)

Breathlessness or chest pain

Laboratory tests

FBC (anaemia)
NT-proBNP (heart failure)
Troponin and CRP (if chest pain suggesting myocarditis or pericarditis, with ECG)if neededThe HAS “Chest pain” page proposes, if in doubt, ECG, troponin and cardiology opinion or a call to the emergency medical service (SAMU); a raised troponin confirms myocardial necrosis (myocarditis or coronary syndrome) and requires calling the cardiologist or 15 ^[13].
D-dimers (if pulmonary embolism suspected with a Wells score ≤ 4)if needednon-French guidelineThe HAS asks for pulmonary embolism to be ruled out in dyspnoea that does not improve, without specifying the means ^[14]; American Family Physician considers D-dimer in this work-up ^[5]. NICE reserves it for a Wells score of 4 points or less ^[15]. It notes more false positives during Covid-19 ^[15].

Detailed explanation and international comparison (in French): read it in English (machine translation)

Persistent pain or myalgia

Laboratory tests (complete the first-line tests, do not repeat them)

FBC with platelets
CRP
Liver function tests (AST, ALT, GGT, alkaline phosphatase)
Serum creatinine with eGFR
Serum calcium
TSH
Serum protein electrophoresis
CK (myalgia, muscle fatigue)if neededThe HAS “Pain” page places it in the further tests guided by the clinical picture; the “Neurological manifestations” page considers muscle enzyme measurement useful in myalgia, checking that the patient is not on a statin ^[17,18].
Antinuclear antibodies (ANA)if neededOnly if autoimmune disease is clinically suspected (arthritis, skin signs, Raynaud…) ^[17].
Anti-dsDNA antibodiesif neededOnly if autoimmune disease is clinically suspected (arthritis, skin signs, Raynaud…) ^[17].
Anti-ENA antibodiesif neededOnly if autoimmune disease is clinically suspected (arthritis, skin signs, Raynaud…) ^[17].
ANCAif neededOnly if autoimmune disease is clinically suspected (arthritis, skin signs, Raynaud…) ^[17].
Cryoglobulinsif neededOnly if autoimmune disease is clinically suspected (arthritis, skin signs, Raynaud…) ^[17].

Detailed explanation and international comparison (in French): read it in English (machine translation)

Persistent skin lesions

Depending on the lesion

FBC (chilblain-like lesions, livedo racemosa)if neededChilblain-like lesions: FBC and antinuclear antibodies first-line; livedo racemosa: FBC, PT, aPTT, electrolytes, HIV and HCV serology, testing for antiphospholipid syndrome ^[19].
Antinuclear antibodies with titre and specificity, including anti-centromere and anti-Scl70 (chilblain-like lesions, Raynaud phenomenon)if neededFirst-line for chilblain-like lesions (titre and specificity) or Raynaud's phenomenon (anti-centromere and anti-Scl70 specificities) ^[19].
Platelet count and haemostasis: PT, aPTT (spontaneous bruising)if neededSpontaneous bruising described in prolonged forms requires a platelet count and haemostasis tests ^[19].

Detailed explanation and international comparison (in French): read it in English (machine translation)

Sources (19)