What biochemical tests should I consider for a patient presenting with unexplained fatigue and weight loss?

Guideline-aligned answer with reasoning, red flags and references. Clinically reviewed by Dr Kola Tytler MBBS CertHE MBA MRCGP.

Posted: 22 August 2025Updated: 22 August 2025 Guideline-Aligned (High Confidence) Clinically Reviewed
Dr Kola Tytler MBBS CertHE MBA MRCGPClinical Lead • iatroX

For a patient presenting with unexplained fatigue and weight loss, the following biochemical tests should be considered in primary care:

  • Full blood count (FBC) urgently within 48 hours to assess for possible leukaemia or anaemia .
  • Serum CA125 measurement in women aged 18 and over to investigate possible ovarian cancer .
  • Blood glucose levels to identify new-onset diabetes, especially in patients aged 60 and over, which may indicate pancreatic cancer .
  • Serum protein electrophoresis or plasma viscosity/erythrocyte sedimentation rate if myeloma is suspected, particularly with bone pain or unexplained fractures .
  • Consider quantitative faecal immunochemical testing if iron-deficiency anaemia is present, which may suggest colorectal cancer .

Additionally, an urgent chest X-ray should be offered within 2 weeks for patients aged 40 and over with unexplained fatigue and weight loss who have a history of smoking or asbestos exposure, or if respiratory symptoms are present, to exclude lung or mesothelioma .

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Educational content only. Always verify information and use clinical judgement.