Confirmed or Probable SARS-CoV-2 Infection
Diagnosis starts from a documented COVID-19 infection episode.
- 2021: WHO definition year (Delphi consensus)
- PCR/Antigen: Confirmed test types (lab-verified)
- Clinical/epi: Probable case basis (no test needed)
- ~65M: Global long COVID est. (estimated cases)
What counts as index infection
Confirmed cases use a positive PCR or antigen test.
Probable cases rely on typical symptoms plus exposure history when testing was unavailable.
Both confirmed and probable infections qualify as the diagnostic starting point.
Why the anchor point matters
Every subsequent criterion — onset window, duration — is measured from this date.
An inaccurate index date can wrongly include or exclude a patient.
Population scale
Post-COVID condition affects a meaningful share of prior infections.
Estimates vary widely by cohort, severity, and vaccination status.
Symptom Onset Within 3 Months
New or persisting symptoms must begin within a 3-month window.
- ≤3 mo: Onset window (from infection)
- 2–6 wk: Typical onset (most common)
- Rare: Late-onset cases (still qualifying)
- WHO ICD-10: Window origin (U09.9 code)
Defining the window
The 3-month cutoff distinguishes post-COVID condition from unrelated new illness.
Symptoms can be continuous from acute infection or emerge after a symptom-free interval.
Onset beyond 3 months typically requires reconsidering alternative causes.
Continuous vs relapsing onset
Some patients never fully recover from acute illness.
Others feel better, then symptoms return within the window.
Tracking practicalities
Clinicians document exact onset dates relative to infection.
Symptom diaries improve recall accuracy for retrospective cases.
Symptoms Persisting At Least 2 Months
Qualifying symptoms must last a minimum of 2 months.
- ≥2 mo: Minimum duration (to qualify)
- ~9 mo: Median duration (reported cohorts)
- Common: Fluctuating course (waxing/waning)
- Periodic: Reassessment interval (clinical follow-up)
Why 2 months
The threshold filters out short post-viral recovery periods.
It separates typical convalescence from a persistent condition.
Symptoms under 2 months do not meet post-COVID condition criteria.
Fluctuation and relapse
Symptoms often wax and wane rather than staying constant.
Overall persistence across the window is what counts.
Ongoing monitoring
Duration is reassessed at follow-up visits.
Resolution before 2 months rules out the diagnosis.
Exclusion of Alternative Diagnoses
Symptoms must not be better explained by another condition.
- Multiple: Common mimics (anemia, thyroid, etc.)
- Targeted: Workup scope (symptom-guided)
- Frequent: Overlap conditions (ME/CFS, POTS)
- Yes: Diagnosis of exclusion (clinical judgment)
Ruling out mimics
Labs and exams screen for anemia, thyroid disease, and other explainable causes.
Only unexplained symptoms count toward diagnosis.
Alternative-diagnosis exclusion prevents misclassifying other treatable conditions.
Overlapping syndromes
ME/CFS and POTS often co-occur with post-COVID condition.
They may coexist rather than exclude the diagnosis.
Clinical judgment
Exclusion relies on reasonable clinical assessment, not exhaustive testing.
Unnecessary testing burden is discouraged by WHO guidance.
Diagnosis & Phenotype Classification
Met criteria yield a diagnosis and a dominant symptom phenotype.
- ICD-10 U09.9: Diagnosis code (post-COVID condition)
- 4 clusters: Common phenotypes (fatigue, resp, cog, cardio)
- Fatigue: Most reported cluster (across cohorts)
- Common: Multi-phenotype overlap (mixed presentations)
Confirming the diagnosis
All three criteria — onset, duration, exclusion — must be satisfied together.
Missing any one criterion means no formal diagnosis yet.
Meeting all three criteria yields a formal post-COVID-19 condition diagnosis.
Phenotype clusters
Fatigue-dominant, respiratory-dominant, cognitive-dominant, and cardiovascular-dominant clusters are common.
Phenotyping guides symptom-targeted management.
Why phenotypes matter
Distinct clusters may reflect different underlying mechanisms.
Phenotype-aware care improves symptom-specific treatment planning.