Which specific blood tests are most useful in the workup of PUO, and how should I interpret their results?

Specific blood tests useful in the workup of pyrexia of unknown origin (PUO) include:

• Full blood count (FBC) to assess for leukocytosis, anemia, or thrombocytosis, which may indicate infection, inflammation, or malignancy.
• C-reactive protein (CRP) as a marker of inflammation or infection.
• Blood cultures to identify any bloodstream infections.
• Serum protein electrophoresis and serum free light chain testing if myeloma is suspected, especially with persistent bone pain or unexplained fractures.
• Serum CA125 in women aged 18 and over if ovarian cancer is a consideration.

Interpretation of results:

• An elevated white cell count may suggest infection or hematological malignancy; a very urgent FBC is recommended within 48 hours if leukemia is suspected in adults with unexplained fever.
• Raised CRP supports the presence of an inflammatory or infectious process but is nonspecific.
• Positive blood cultures confirm bloodstream infection and guide antibiotic therapy.
• Serum protein electrophoresis abnormalities and elevated free light chains suggest myeloma and warrant urgent referral.
• CA125 ≥35 IU/ml in women should prompt an ultrasound scan of the abdomen and pelvis to investigate for ovarian cancer.

These tests should be selected based on clinical context and suspicion, and results interpreted in conjunction with clinical findings to guide further investigation or referral.

References: ¹,²