It was overuse of acetaminophen that finally led to Nic Petermann’s cancer diagnosis. For months, the then 26-year-old had been contending with exhaustion, night sweats, recurring fevers, and abdominal pain so debilitating that she regularly woke up in the middle of the night to take soothing baths. Her persistent flu-like symptoms, she’d read online, were probably just the lingering effects of a Covid infection she’d had in January 2021; the pain was the odd symptom out, but an ultrasound had turned up nothing.

Come June, the pain was too much to bear—Petermann called a telehealth hotline and was immediately referred to the hospital after the staff heard how much acetaminophen she had been taking. After extensive testing, Petermann finally had an answer: All her symptoms, including those that seemed to be long Covid, were due to Stage IV Hodgkin’s lymphoma. She started chemotherapy the next day.

Today, Petermann is in remission, though she still deals with the long-term consequences of the aggressive, monthslong chemo. If she hadn’t assumed most of her symptoms were due to long Covid, she says, she may have received proper treatment and a diagnosis much earlier. “When I went to get my pain symptoms checked out, I didn’t mention the flu-like symptoms, because I just thought that was something that I would have to deal with,” she says.

Most people with Petermann’s symptoms won’t end up in her position. Long Covid is common—estimates of its prevalence vary widely, but even the most conservative studies imply that millions of people are dealing with long-lasting symptoms of their infections. Hodgkin’s lymphoma, on the other hand, is rare. But with dozens of possible symptoms, long Covid can be easily confused with countless other conditions, including cardiovascular diseases such as hypertension and diabetes, autoimmune diseases like lupus and multiple sclerosis, and cancer. Add the fact that Covid can make preexisting conditions worse, and determining whether or not someone has long Covid becomes a daunting task.

Parsing these vast sets of alternatives has become the responsibility of clinicians on the vanguard of long Covid care, from the primary care physicians whom patients first seek out to the experts who staff long Covid clinics. For each patient they must perform a careful differential diagnosis, a medical term for the process of considering every possible cause of a patient’s set of symptoms.

Accurate differential diagnosis is essential not just for getting patients care, but also for furthering medical understanding of a still-obscure condition. “We need to be cautious not to turn long Covid into a catch-all diagnosis,” says Linda Geng, codirector of the Stanford Post-Acute Covid-19 Syndrome Clinic.

In the absence of any objective tests, however, long Covid remains a “diagnosis of exclusion”—one that is made only after other reasonable possibilities have been exhausted. Recent data suggest that many patients will emerge from this process with a diagnosis not of long Covid, but of something else. A July paper in Nature that analyzed the medical records of over 2 million patients in the UK found that, while 5.4 percent of those with a previous Covid infection had at least one long Covid symptom recorded in their charts, 4.5 percent without evidence of infection also had at least one symptom.