Sleep and Rhythmic Stimulation

urinary excretion was not further considered as a possible diagnostic test. We feel that the more laborious faecal collections 'have certain theoretical advantages over the suggested technique of a 24-hour urine collection. Firstly, the faecal estimation is based on the simple assumption that the unabsorbed labelled fat is directly measurable, whereas the method based on urine depends partly on other variables, such as an unknown proportion of the ingested fat which may be laid down in the fat depots or even an incompletely blocked thyroid gland. Since urine measurements are an indirect means of ascertaining the average level of inorganic radio-iodine present in the plasma during the period of collection, the values obtained will depend on the rate at which the radioactive iodine is split off from the fat by metabolic processes. Secondly, faecal collection avoids the need for having to pay regard to renal function. Thirdly, as pointed out by Lubran and Pearson,' by having the patient on a known fat-intake a chemical fat balance may be performed if equivocal results are obtained without the delay entailed by a further collection of specimens. On the other hand, the suggested dose of 1 1.C for the urine method is lower than the 5 ,uC used by us in adult cases. Furthermore, although faecal and urine admixture would invalidate both methods, faecal contamination of urine is recognized more easily. As collection of a 24-hour urine is simpler and quicker than a 5-day faecal collection, the further experience of Mr. Walker and his colleagues with their test will be awaited with interest, especially in respect of patients in whom there is only slight or moderate fat malabsorption when the diagnostic value of the fat-absorption test is at its greatest.-We are, etc., P. D. GRIFFITHS. R. G. HUNTSMAN. Department of Clinical Pathology, A. R. H. WORSSAM. Guy's Hospital, London, S.E. 1. REFERENCES Lubran, M., and Pearson, J. D., J. clin. Path., 1958, 11, 165. V2eall, N., and Vetter, H., Radioisotope Techniques in Clinical Research and Diagnosis, 1958. Butterworth.