Fatigue: which lab tests are useful – and when more testing does not help

For primarily unexplained fatigue, the DEGAM S3 guideline recommends a limited basic laboratory work-up: blood glucose, a full blood count, ESR or CRP, transaminases or γ-GT, and TSH.

Further laboratory or technical investigations should follow abnormal findings or specific clues from the medical history and examination rather than being added as a broad screening panel.

Additional tests become more useful when there is a concrete reason for them. Examples include blood loss or other clues to iron deficiency, restricted diet or malabsorption, neurological symptoms, relevant medicines, or other risk factors.

Depending on that context, ferritin, vitamin B12, folate, vitamin D or other investigations may be appropriate. The point is to match the test to the clinical question rather than to infer a cause from fatigue alone.

That is possible. Normal basic laboratory results make some common or treatable explanations less likely, but they do not explain every form of fatigue or exertion intolerance and do not invalidate persistent symptoms.

The next step in clinical assessment is guided by the course over time, sleep, medicines, infection history, physical and mental-health symptoms, examination findings and any new or changing features.

NICE states that there is currently no diagnostic test for ME/CFS. When ME/CFS is suspected, laboratory investigations are used to identify or exclude alternative and coexisting conditions.

A delayed and disproportionate worsening of symptoms after activity is part of the clinical history of post-exertional malaise. It should not be inferred from a laboratory value, and a normal routine panel does not rule it in or out.

New, pronounced or persistent fatigue deserves medical assessment when it substantially limits daily life or is accompanied by concerning features such as shortness of breath, chest pain, fainting, fever, unintended weight loss, bleeding or rapid deterioration.

Laboratory testing is one part of that assessment. The appropriate work-up follows the overall clinical picture rather than a fixed online checklist.