There is no single test that confirms long COVID. Diagnosis relies on a combination of your symptom history, a physical exam, and lab work to rule out other conditions that could explain how you’re feeling. Both the WHO and CDC define long COVID as symptoms lasting at least three months after a COVID-19 infection that can’t be explained by another diagnosis. If you suspect you have it, understanding what the diagnostic process looks like can help you advocate for the right workup.
What Qualifies as Long COVID
The WHO defines long COVID (formally called “post-COVID-19 condition”) as symptoms that appear within three months of a confirmed or probable SARS-CoV-2 infection, persist for at least two months, and aren’t explained by something else. The CDC uses a similar framework: symptoms present for at least three months that may be continuous, come and go, or progressively worsen.
Most people recover from acute COVID within four weeks, and many continue improving between weeks four and twelve. Long COVID is the term used when symptoms either don’t resolve after that window or new symptoms appear. These can affect virtually any organ system and range from fatigue and brain fog to heart palpitations, shortness of breath, and joint pain. Symptoms don’t have to be constant. A pattern of flare-ups and remissions still counts.
Why There’s No Single Diagnostic Test
No validated blood biomarker for long COVID exists in clinical practice today. A 2024 NIH-funded study ran 25 standard blood and urine tests on participants with and without long COVID and found that routine lab results could not reliably distinguish between the two groups. That means your complete blood count, metabolic panel, and other common labs may all come back normal, even when your symptoms are very real.
Researchers are making progress. One recent study identified a panel of proteins in blood plasma that could differentiate long COVID patients with neurological symptoms from healthy controls with up to 94% accuracy. But these panels use specialized proteomics platforms not yet available in standard clinical labs. For now, diagnosis remains a clinical judgment call, not a lab result.
What a Typical Workup Looks Like
Your doctor will start by taking a detailed history: when you had COVID (confirmed by test or strongly suspected), what symptoms appeared afterward, and how they’ve changed over time. Expect questions about fatigue levels, sleep quality, exercise tolerance, cognitive function, mood changes, and any new symptoms you didn’t have before your infection.
A physical exam comes next, followed by lab work. The labs aren’t designed to confirm long COVID. They’re meant to rule out other conditions that produce similar symptoms. Your provider may order some combination of a complete blood count, metabolic panel, thyroid function tests, hemoglobin A1c (to check blood sugar regulation), urinalysis, and inflammatory markers. If these come back abnormal, your doctor will investigate whether another condition is responsible. If they come back normal and your symptoms align with a post-COVID timeline, that pattern itself supports a long COVID diagnosis.
Testing for Brain Fog and Cognitive Issues
Brain fog is one of the most common and disabling long COVID symptoms, and it can be formally measured. The Montreal Cognitive Assessment, or MoCA, is a 30-point screening tool that evaluates memory, attention, language, and reasoning. A score below 26 suggests cognitive impairment. In one study comparing post-COVID patients to matched controls, 66.7% of the post-COVID group scored below that threshold, compared to just 15% of controls. The average score for post-COVID patients was 24.1, versus 27.3 in healthy participants.
Beyond the MoCA, clinicians may use timed tests like the Trail Making Test, which measures processing speed and the ability to switch between tasks. Post-COVID patients in the same study took significantly longer to complete both versions of this test: about 44 seconds versus 31 seconds on the simpler version, and 96 seconds versus 77 seconds on the more complex one. A digit span test, where you repeat number sequences forward and backward, can reveal problems with working memory. These aren’t exotic neurological exams. They’re quick, office-based screenings that give your provider objective data to pair with your subjective experience of “feeling foggy.”
Evaluating Exercise Intolerance
If your main complaint is that you can’t exercise or exert yourself the way you used to, your doctor may refer you for cardiopulmonary exercise testing, or CPET. This involves exercising on a bike or treadmill while wearing a mask that measures your oxygen consumption and carbon dioxide output in real time.
Long COVID patients often show a distinctive pattern on CPET: their bodies are less efficient at using oxygen. One study found that post-COVID patients had significantly impaired oxygen extraction at the tissue level compared to controls, meaning their muscles weren’t pulling oxygen from the blood as effectively. They also showed greater ventilatory inefficiency, requiring more breathing effort relative to the carbon dioxide they produced. Standard lung function tests like spirometry may look completely normal in these same patients, which is why CPET can be valuable. It catches problems that only show up under exertion.
Screening for POTS and Heart Rate Issues
A surprisingly common finding in long COVID is postural orthostatic tachycardia syndrome, or POTS. This is a condition where your heart rate spikes excessively when you stand up, causing dizziness, lightheadedness, rapid heartbeat, and sometimes fainting. The diagnostic criteria are specific: a heart rate increase of at least 30 beats per minute when moving from lying down to standing, confirmed by either an active standing test or a tilt table test, without a corresponding drop in blood pressure. Symptoms must have been present for at least three months, and other causes need to be excluded.
If you notice your heart racing when you stand, feel dizzy in the shower, or can’t tolerate being upright for long periods, mention this to your doctor. POTS is treatable, and identifying it can open the door to specific management strategies that improve daily functioning. Your provider may start with a simple in-office standing test before referring you to a cardiologist for formal tilt table testing and 48-hour heart rhythm monitoring.
Getting a Formal Diagnosis on Your Record
Long COVID has its own medical billing code: U09.9 in the ICD-10 system. Your doctor can assign this code if you have a history of confirmed or probable COVID-19 and are presenting with post-COVID symptoms. They’ll also code the specific symptoms or conditions you’re experiencing, such as fatigue, cognitive impairment, or shortness of breath. Having this code on your medical record matters for insurance coverage, disability documentation, and ensuring continuity of care if you see multiple specialists.
Because there’s no definitive lab test, some patients encounter providers who are skeptical or unfamiliar with long COVID. If your symptoms started after a COVID infection, have persisted for three months or more, and don’t have another clear explanation, you meet the clinical definition. Seeking care at a post-COVID clinic, if one is accessible to you, can streamline the process. These clinics are set up to coordinate the multi-system evaluation that long COVID often requires, pulling together pulmonology, cardiology, neurology, and rehabilitation under one roof.