OSCE — Methaemoglobinaemia

Brief (to candidate)

You will assess a patient with a presentation consistent with Methaemoglobinaemia.
You have 8 minutes to take a focused history, outline examination, investigations, and management including red flags.

Clinical context

Methaemoglobinaemia is the presence of methaemoglobin (MetHb) — haemoglobin in which the haem iron is in the ferric (Fe3+) state (normal haemoglobin is ferrous, Fe2+) — which cannot bind or transport oxygen and, worse, shifts the oxygen-haemoglobin dissociation curve to the LEFT, so that oxygen bound to neighbouring normal haemoglobin subunits is held more tightly and released less readily to tissues. Normal MetHb is under 1 percent of total haemoglobin; levels over 1.5 percent are abnormal. Two mechanisms: ACQUIRED (oxidant drugs/chemicals — over 99 percent of cases) — nitrates/nitrites (contaminated well water in infants, sodium nitrite food preservative, amyl/sodium/butyl nitrite 'poppers

Candidate tasks

1. Clarify onset, severity, associated features, and red-flag symptoms.
2. State focused examination priorities.
3. List first-line investigations and any named score/criteria.
4. Give immediate resuscitation steps.
5. Outline definitive management with doses/routes where standard.
6. Name complications and disposition (ward / HDU / theatre / discharge safety-net).
7. Mention one special-population modifier (pregnancy, child, elderly, CKD).

Examiner checklist

Model outline

Lead with the working diagnosis and life threats. Resuscitate before definitive tests when unstable. Use guideline-standard therapy with named agents and doses. Document escalation criteria and follow-up. A safe candidate is specific, structured, and never delays critical care for non-urgent imaging.