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 kU^A/l compared with 620 kU^A/l, as calculated from diluted samples.