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CorrespondenceReferring to the specificity of the test, Dr Gottfried is surprised that AGLT is positive not only in HS but also in other conditions, in particular chronic renal failure, myelofibrosis and chronic leukaemias; in fact, in these conditions an increase in red cell osmotic fragility is not uncommon (Dacie, 1963; Miale, 1977), when looked for by appropriate tests (not including, of course, the 'standard' glycerol lysis test which is negative even in most of HS patients); most of the non-HS AGLT-positive cases we have found were also positive when tested by the NaCl osmotic fragility test on incubated blood and by the 'alternate' glycerol lysis test.Referring to the two last comments, the lower limit for normal subjects has been deliberately maintained high (pH 6.85 is critical from this point of view) to increase the gap between normal and abnormal and to obtain clear-cut results, not obtainable by both the standard and the alternate methods. As far as the need for '30 min continuous observation' is concerned, it should be noted that almost all the HS patients showed AGLT,, values in less than 150 s.In conclusion, AGLT seems to be the best screening test currently available since it has 100% sensitivity (evaluated on 73 HS cases), is specific, gives clear-cut results and does not require sterile blood.The only drawback of this method is that the experimental conditions are very critical, in particular the pH of the phosphate buffer and the mixing of the red cell/glycerol suspension.

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