• Abnormal sensitivity of RBCs from patients with PNH to the haemolytic action of complement.

• Complement is activated by acidification of patient's serum to pH of 6.2 which induces lysis of PNH red cells but not normal controls.

Specificity: high—similar reaction is produced only in the rare syndrome HEMPAS (a form of congenital dyserythropoietic anaemia type II) which should be easily distinguished morphologically.

Sensitivity: low—as the reaction is crucially dependent on the concentration of magnesium in the serum.

It appears to be a technically difficult test in most laboratories. Patients with only a low % of PNH cells may be missed at an early stage of the disease. Markedly abnormal PNH cells are usually picked up in ~75% of patients. Less abnormal cells are detected in only ~25% of patients.

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