2-day CPET
A 2-day CPET is a cardiopulmonary exercise test given on two successive days to measure the effect of post-exertional malaise (PEM) on a patient's ability to exercise. PEM is a cardinal symptom of myalgic encephalomyelitis/chronic fatigue syndrome and is common in long COVID as well.

Background 

Several differences have been found between people with and without ME/CFS, including people with other diseases or who are sedentary. On the first test, people with ME/CFS exhibit lower performance and heart rate, and on the second test, performance is even lower, while for controls, it is the same or slightly better. The largest decrease is in anaerobic threshold, which signifies a shift from aerobic to anaerobic metabolism at a lower level of exertion, and is not influenced by effort. Peak power output, heart rate, and VO2max also decrease, and in ME/CFS, but effort and familiarity with the test may affect VO2max and power. Additionally, healthy people generally recover from a CPET within 24 hours while people with ME/CFS do not. As such, a 2-day CPET can objectively measure PEM and its effect on physical functioning. However, its utility has not been completely confirmed, as many studies of it have been small. While it should not be required for a diagnosis, a 2-day CPET can show that symptoms are not due to deconditioning and provide evidence for obtaining disability benefits. Because PEM is also a symptom of long COVID, the 2-day CPET may be useful in evaluating exercise intolerance there as well.

The cause of decreased performance is not understood. Proposals include impaired oxygen transport, impaired aerobic metabolism, and mitochondrial dysfunction.

The test provokes symptoms by design, and recovery may be prolonged. In some cases, it may worsen the illness long-term.

References 

Medical tests
Chronic fatigue syndrome
Long COVID
Exercise physiology