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PubMed This is a summary of 18 peer-reviewed journal articles Updated
Primary Care · Post-Acute Sequelae of SARS-CoV-2

Diagnosing Long COVID and Recognizing Red Flags

At a Glance

Long COVID is diagnosed from the pattern of symptoms after COVID-19 infection, a medical history and examination, and tests that rule out other conditions; no single blood test or scan confirms it. Sudden chest, breathing, neurologic, or clot symptoms need emergency care.

Getting a diagnosis for Long COVID—medically known as Post-Acute Sequelae of SARS-CoV-2 (PASC) — can be a frustrating process because there is no single blood test or scan that can “prove” you have it [1]. Instead, doctors reach a clinical diagnosis by looking at your medical history and carefully ruling out other conditions that could explain your symptoms [2][3].

The Diagnostic Process

A research diagnosis of Long COVID is typically made using the WHO definition of symptoms that usually begin within 3 months of the infection and last for at least 2 months [1]. However, you do not have to wait a rigid amount of time to seek clinical evaluation.

Because Long COVID can affect almost every organ system, your doctor’s first job is to ensure your symptoms aren’t being caused by a different, treatable condition. A normal test result does not disprove Long COVID; it simply rules out specific alternative causes [1][2].

Ruling Out “Mimic” Conditions

Your care team will likely run targeted tests. Common conditions that can look like Long COVID include:

  • Endocrine Issues: Thyroid disorders (hypothyroidism or hyperthyroidism) and new-onset diabetes can cause fatigue, heart palpitations, and weight changes [4][5].
  • Blood Disorders: Anemia (low iron or red blood cells) and vitamin deficiencies (like B12 or Vitamin D) are frequent causes of profound exhaustion and “brain fog” [3].
  • Organ-Specific Damage: Your doctor may check for underlying heart disease, chronic lung issues, or kidney dysfunction that may have been triggered or worsened by the virus [4][6].
  • Sleep and Mental Health Disorders: Conditions like sleep apnea, depression, or anxiety can coexist with Long COVID and worsen daytime fatigue [3].

When to Seek Emergency Care: Red Flags

While most Long COVID symptoms fluctuate, the virus can sometimes lead to serious complications such as blood clots, heart inflammation, or strokes [7][8].

If you experience any of the following “red flag” symptoms, do not drive yourself. Seek immediate medical evaluation at an emergency department:

  • Cardiovascular Signs: New, severe, or “crushing” chest pain or pressure; or fainting (syncope) [9][10].
  • Respiratory Signs: Sudden, severe shortness of breath; or coughing up blood [8][11].
  • Neurologic Signs: Sudden weakness or numbness (especially on one side of the body), facial drooping, difficulty speaking, new confusion, or a new, “worst-ever” headache [7].
  • Vascular Signs: Sudden swelling, redness, and pain in one leg, which could indicate a deep vein thrombosis (DVT) that could travel to the lungs (pulmonary embolism) [8][12].

Tailored Diagnostic Tests

While your workup will be tailored to your specific symptoms and examination, an evaluation often includes:

  1. Basic Lab Work: Complete Blood Count (CBC) and comprehensive metabolic panels [13][6].
  2. Heart Evaluation: An Electrocardiogram (ECG) to check your heart’s rhythm. Further testing like an echocardiogram is directed by a specialist if symptoms warrant it [14][9].
  3. Lung Evaluation: Pulmonary Function Tests (PFTs) to see how well your lungs move air if you have shortness of breath [15][16].
  4. Autonomic Testing: An in-office “stand test” or “tilt-table test” if you experience dizziness or a racing heart when you stand up [17][18].

Common questions in this guide

How is Long COVID diagnosed if there is no single test?
Long COVID is diagnosed clinically using your COVID-19 history, symptoms, examination, and targeted testing. Tests are mainly used to find or rule out other treatable conditions; a normal result does not by itself rule out Long COVID.
How long after COVID-19 do symptoms need to last for Long COVID?
The WHO research definition generally describes symptoms that start within 3 months of infection and last at least 2 months. You do not need to wait a fixed period before asking a clinician to assess new or ongoing symptoms.
What conditions can mimic Long COVID?
Thyroid problems, diabetes, anemia, vitamin deficiencies, heart or lung disease, kidney dysfunction, sleep apnea, depression, and anxiety can cause or worsen similar symptoms. Your clinician may order targeted tests based on your symptoms.
What tests might be part of a Long COVID workup?
An evaluation may include a complete blood count and metabolic panel, with an ECG to check heart rhythm when heart symptoms are present. Shortness of breath may lead to lung function tests, while dizziness or a fast heart rate on standing may call for a stand test or tilt-table test.
When should I go to the emergency department for Long COVID symptoms?
Get immediate emergency care for severe or crushing chest pain, fainting, sudden severe shortness of breath, coughing up blood, or possible stroke symptoms such as one-sided weakness, facial drooping, trouble speaking, confusion, or a sudden worst-ever headache. Sudden swelling, redness, and pain in one leg can signal a blood clot; do not drive yourself.
Can palpitations or dizziness when standing be evaluated for Long COVID?
Yes. A clinician can review your heart rate and blood pressure while you change position and may use an office stand test or tilt-table test. An ECG and other heart tests may also be considered when appropriate.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Which specific conditions, such as thyroid dysfunction, anemia, or sleep disorders, should we rule out through testing?
  2. 2.Since I have chest pain and palpitations, should we consider further cardiac evaluation?
  3. 3.Do my lung function tests explain my shortness of breath, or should we look for other vascular or lung issues?
  4. 4.Can you review my heart rate and blood pressure while lying down and standing safely in the office?
  5. 5.At what point should I be referred to a multidisciplinary clinic or a specialist like a cardiologist or neurologist?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (18)
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    Long COVID or Post-Acute Sequelae of SARS-CoV-2 Infection (PASC) and the Urgent Need to Identify Diagnostic Biomarkers and Risk Factors.

    Parums DV

    Medical science monitor : international medical journal of experimental and clinical research 2024; (30()):e946512 doi:10.12659/MSM.946512.

    PMID: 39289865
  2. 2

    Post COVID fatigue: Can we really ignore it?

    Sharma P, Bharti S, Garg I

    The Indian journal of tuberculosis 2022; (69(2)):238-241 doi:10.1016/j.ijtb.2021.06.012.

    PMID: 35379408
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    Clinical Characteristics of Patients Initially Suspected of Long COVID: A Case Series From a Specialized Outpatient Clinic.

    Ohira M, Osada T, Kimura H, et al.

    Journal of general and family medicine 2026; (27(1)):e70081 doi:10.1002/jgf2.70081.

    PMID: 41696720
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    The Aftermath of COVID-19: Exploring the Long-Term Effects on Organ Systems.

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    Biomedicines 2024; (12(4)) doi:10.3390/biomedicines12040913.

    PMID: 38672267
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    Beyond the acute illness: Exploring long COVID and its impact on multiple organ systems.

    Bhattacharjee N, Sarkar P, Sarkar T

    Physiology international 2023; (110(4)):291-310 doi:10.1556/2060.2023.00256.

    PMID: 37943302
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    Multidisciplinary Center Care for Long COVID Syndrome-A Retrospective Cohort Study.

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    The American journal of medicine 2025; (138(1)):108-120 doi:10.1016/j.amjmed.2023.05.002.

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    Pathogenic mechanisms of post-acute sequelae of SARS-CoV-2 infection (PASC).

    Sherif ZA, Gomez CR, Connors TJ, et al.

    eLife 2023; (12()).

    PMID: 36947108
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    Excess burden of respiratory and abdominal conditions following COVID-19 infections during the ancestral and Delta variant periods in the United States: An EHR-based cohort study from the RECOVER program.

    Varma JK, Zang C, Carton TW, et al.

    PloS one 2024; (19(6)):e0282451 doi:10.1371/journal.pone.0282451.

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    Post-Acute COVID-19 Syndrome and the cardiovascular system: What is known?

    Dixit NM, Churchill A, Nsair A, Hsu JJ

    American heart journal plus : cardiology research and practice 2021; (5()):100025 doi:10.1016/j.ahjo.2021.100025.

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    The other COVID-19 survivors: Timing, duration, and health impact of post-acute sequelae of SARS-CoV-2 infection.

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    Journal of clinical nursing 2024; (33(1)):76-88 doi:10.1111/jocn.16541.

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    Long COVID syndrome: exploring therapies for managing and overcoming persistent symptoms.

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    Inflammopharmacology 2025; (33(7)):4097-4113 doi:10.1007/s10787-025-01807-w.

    PMID: 40622467
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    Long COVID and Wavering Incidence of Pulmonary Embolism: A Systematic Review.

    Shah B, Ahmad MN, Khalid M, et al.

    Journal of community hospital internal medicine perspectives 2023; (13(5)):23-31 doi:10.55729/2000-9666.1233.

    PMID: 37868668
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    The Female-Predominant Persistent Immune Dysregulation of the Post-COVID Syndrome.

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    Mayo Clinic proceedings 2022; (97(3)):454-464 doi:10.1016/j.mayocp.2021.11.033.

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    Cardiovascular symptom phenotypes of post-acute sequelae of SARS-CoV-2.

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    A Retrospective Observational Study of Pulmonary Impairments in Long COVID Patients.

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    Postural orthostatic tachycardia syndrome as a sequela of COVID-19.

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    Post-COVID dysautonomias: what we know and (mainly) what we don't know.

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This page is for informational purposes only and does not constitute medical advice. A clinician should evaluate your symptoms, and sudden or severe red-flag symptoms require emergency care.

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