Fig. 4. Hook effect observed in a patient with a positive suxamethonium intradermal test. The observed response (fluorometric units) is plotted against the calculated quaternary ammonium morphine–immunoglobulin E (IgE) concentration in serially diluted serum. IgE concentration in undiluted serum, read from the calibration curve, was 41 kUA/l compared with 620 kUA/l, as calculated from diluted samples.