Introduction: Who Should Seek Diagnostic Evaluation

If you had COVID-19 and your symptoms have continued for more than four weeks—or if new symptoms have developed after you thought you had recovered—it may be time to talk to your doctor about the possibility of long COVID. This condition, officially known as post-acute COVID-19 syndrome, can affect anyone who has been infected with the virus that causes COVID-19, regardless of how mild or severe their initial illness was.^[1]

You should consider seeking diagnostic evaluation if you are experiencing persistent fatigue, difficulty thinking clearly (often called “brain fog”), shortness of breath, chest pain, changes in your sense of taste or smell, or any other symptoms that interfere with your daily activities and quality of life. These symptoms can emerge days after your initial COVID-19 diagnosis, persist from the original illness, or even appear weeks later in people who never realized they were infected.^[3]

Long COVID does not discriminate. While it appears more common in people who had severe COVID-19 illness requiring hospitalization, it can also develop in those who had mild symptoms or were completely asymptomatic during their acute infection. Children can develop long COVID too, though it seems to be less common in younger populations than in adults.^[3]

Certain groups of people may be at higher risk for developing long COVID and should be particularly attentive to persistent symptoms. Women appear more likely to develop long COVID than men. Hispanic and Latino individuals, people with underlying health conditions, adults who are 65 or older, and those who were not vaccinated against COVID-19 face elevated risk. If you fall into any of these categories and notice ongoing symptoms, consulting with a healthcare provider is advisable.^[3]

Classic Diagnostic Methods for Post-Acute COVID-19 Syndrome

Diagnosing long COVID presents unique challenges because, unlike many other conditions, there is no single laboratory test or imaging study that can definitively confirm you have it. Instead, doctors rely on a comprehensive approach that examines your medical history, symptoms, and the results of various tests to rule out other conditions and understand what is happening in your body.^[3]

When you visit your healthcare provider with concerns about possible long COVID, they will typically begin with a detailed conversation about your health history. They will ask about your initial COVID-19 infection—whether you tested positive, what symptoms you experienced, how severe your illness was, and when it occurred. They will also want to know about all the symptoms you are currently experiencing, when they started, how they have changed over time, and how they affect your ability to work, study, or carry out everyday activities.^[4]

A thorough physical examination is an essential part of the diagnostic process. Your doctor will check your vital signs, including blood pressure, heart rate, breathing rate, and body temperature. They will listen to your heart and lungs, examine your body for signs of illness, and assess your overall physical condition. This examination helps identify any obvious abnormalities and guides decisions about which additional tests might be needed.^[6]

Because long COVID can affect many different organ systems in the body, your doctor may order a variety of tests to evaluate how your heart, lungs, kidneys, liver, and other organs are functioning. Blood tests are commonly used to assess overall health and look for signs of inflammation, infection, or organ damage. These might include a complete blood count to check your red and white blood cells, tests to measure kidney and liver function, and markers of inflammation in your body.^[1]

If you are experiencing breathing difficulties or shortness of breath, your doctor may recommend pulmonary function tests. These tests measure how well your lungs are working by assessing how much air you can breathe in and out and how efficiently your lungs transfer oxygen into your bloodstream. Chest X-rays or computed tomography (CT) scans might also be ordered to look at the structure of your lungs and check for any damage or abnormalities left behind by COVID-19.^[7]

For people experiencing chest pain, heart palpitations, or other cardiovascular symptoms, heart-related tests become important. An electrocardiogram (ECG) records the electrical activity of your heart and can detect irregular heart rhythms or other cardiac problems. An echocardiogram, which is an ultrasound of the heart, can show how well your heart is pumping and whether there are any structural issues. Some patients may need additional monitoring through devices that record heart activity over 24 hours or longer.^[12]

Neurological symptoms such as brain fog, memory problems, headaches, or dizziness may require specialized evaluation. While there is no specific test for brain fog, doctors may assess your cognitive function through questionnaires or formal neuropsychological testing. In some cases, brain imaging studies like magnetic resonance imaging (MRI) might be considered, though these are typically used to rule out other conditions rather than to diagnose long COVID itself.^[1]

One important aspect of diagnosing long COVID is distinguishing it from other conditions that can cause similar symptoms. Many symptoms of long COVID—such as fatigue, difficulty concentrating, and mood changes—can also occur in other medical conditions, including thyroid disorders, anemia, depression, or chronic fatigue syndrome. Your doctor will use the combination of your history, physical examination, and test results to determine whether your symptoms are most consistent with long COVID or whether another condition might be responsible.^[4]

For some symptoms, specialized testing may be needed. If you have lost your sense of smell or taste, an ear, nose, and throat specialist might perform specific smell or taste testing. If you are experiencing postural orthostatic tachycardia syndrome (POTS)—a condition where your heart rate increases abnormally when you stand up—specialized autonomic testing can measure how your nervous system controls involuntary functions like heart rate and blood pressure.^[12]

The diagnostic process for long COVID often requires patience and may involve seeing multiple specialists. Because the condition can affect so many different body systems, you might need to consult with cardiologists for heart symptoms, pulmonologists for lung problems, neurologists for neurological issues, or psychiatrists for mental health concerns. A coordinated, multidisciplinary approach helps ensure that all aspects of your condition are properly evaluated and addressed.^[7]

Diagnostics for Clinical Trial Qualification

As researchers work to better understand long COVID and develop effective treatments, clinical trials are being conducted around the world. These research studies test new treatments, medications, or interventions that might help people with long COVID. To participate in a clinical trial, you typically need to meet specific eligibility criteria, which often include particular diagnostic findings or symptom patterns.^[11]

Clinical trials for long COVID usually require documentation that you had a previous SARS-CoV-2 infection. This might be proven through a positive COVID-19 test result from your initial illness, documentation of COVID-19 symptoms with exposure to someone who tested positive, or in some cases, blood tests showing antibodies to SARS-CoV-2. However, the specific requirements vary depending on the trial.^[3]

Most clinical trials define long COVID based on how long symptoms have persisted. The most common definition used in research studies requires that symptoms be present for at least three months after the initial COVID-19 infection. Some trials may require symptoms to have lasted even longer—for example, six months or more. You will need to provide information about when your COVID-19 infection occurred and when your current symptoms began.^[3]

Clinical trials often focus on specific symptoms or symptom clusters. For instance, one trial might be testing a treatment for fatigue and exercise intolerance, while another might focus on cognitive symptoms like brain fog. To qualify for these studies, you may need to demonstrate that you have the specific symptoms being studied, often at a certain level of severity. Researchers might use standardized questionnaires or scales to measure symptom severity and determine if you meet the threshold for participation.^[11]

Baseline testing is a standard part of enrolling in clinical trials. Before you can begin receiving any experimental treatment, researchers need to thoroughly document your current health status. This typically includes comprehensive blood tests, vital sign measurements, and assessments of your organ function. These baseline measurements serve as a comparison point to help researchers understand whether the treatment being tested has any effect on your condition or causes any side effects.^[11]

Some clinical trials may require specific diagnostic test results as part of their enrollment criteria. For example, a trial testing a treatment for lung problems in long COVID might require participants to have abnormal pulmonary function test results or specific findings on chest imaging. A trial focused on cardiovascular symptoms might require evidence of heart rhythm abnormalities or reduced heart function. Understanding what tests you have already had and what they showed can help you and your doctor identify clinical trials for which you might be eligible.^[12]

Exclusion criteria are also important in clinical trials. These are conditions or factors that would prevent you from participating in a study, often for safety reasons. Common exclusions might include having certain other medical conditions, taking specific medications, being pregnant or planning to become pregnant, or having had certain treatments recently. Diagnostic tests may be required to confirm that you do not have any of these exclusionary conditions.^[11]

If you are interested in participating in a long COVID clinical trial, discuss this with your healthcare provider. They can help you understand what documentation and test results you already have, what additional testing might be needed, and how to find trials that match your specific situation. Many specialized long COVID clinics at academic medical centers are actively involved in research and can connect you with ongoing clinical trials.^[12]