A 40-year-old man presents with acute severe headache (thunderclap), nausea, and brief loss of consciousness. CT scan is normal. Lumbar puncture is performed 12 hours after symptom onset. What CSF finding would most specifically confirm subarachnoid haemorrhage (SAH)?

• A Uniformly blood-stained CSF in all four tubes
• B Elevated CSF protein (>1g/L)
• C Xanthochromia on spectrophotometry ✓
• D Elevated opening pressure >20 cmH2O

Explanation

Xanthochromia — yellow discolouration of centrifuged CSF due to oxyhaemoglobin (within 2–4 hours) and bilirubin (developing over 12 hours, persisting up to 2 weeks) — is the most specific finding for SAH when CT is negative. Spectrophotometric detection of bilirubin is the gold standard; visual xanthochromia alone can be missed. Uniformly bloody CSF in all tubes (not clearing) suggests SAH but can also occur with traumatic tap (which clears tube-to-tube). Elevated protein is non-specific. Elevated opening pressure occurs in many conditions and is less specific.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.