To the Editor: We are unable to appreciate the real value in the “simple” test recently reported by Pak and his associates (N Engl J Med 292:497–500, 1975) in differentiating various kinds of hypercalciurias. The entire test is simply a “parathyroid suppression test” in disguise. For most laboratories, measurements of cyclic AMP and immunoreactive parathyroid hormone (iPTH) are equally unavailable. For laboratories, such as ours, that are able to measure iPTH, the cyclic AMP test is certainly no substitute. Although radioimmunoassay of parathyroid hormone may have the inherent problem of immunoheterogeneity (Silverman R, Yalow RS: Heterogeneity of parathyroid hormone: clinical….
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