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Summary The geometrico‐statistical procedures developed in the science of stereology can be applied directly in radiology and nuclear medicine; and their rigorous application will not only result in greater diagnostic accuracy in the interpretation of organ images but also in economies in time, money and effort. The estimation of organ image areas with the method known as “point‐counting” is particularly simple and useful.

SummaryPhylogenetically mammalian platelets are an apocrine secretion from nucleated thrombocytes, and biologically the number of circulating fragments is irrelevant compared with their total mass and biochemical function. The literature on human genetic derangements affecting the circulating platelet mass is reviewed. The product of platelet count and platelet volume is shown to be “canalised” since many congenital forms of thrombocytopenia are associated with a reciprocal increase in the mean platelet volume. Considerations of platelet size are essential since macrothrombocytopenia is a racial characteristic of many Mediterranean peoples and probably also occurs frequently in others races. The location of platelet disorders in relation to the phylogenetic canal provides an objective classification which has diagnostic, therapeutic and prognostic significance.

Summary Definitions from the radiologic and radionuclide literature on “spleen sizing” are reviewed and summarized. Some established relationships are confirmed and the data are extended in a study based on measurements made on abdominal radiographs from 40 healthy adult volunteers, 15 patients with known splenomegaly and 12 patients in whom diagnostic radionuclide images of the spleen were recorded; measurements were also made on 17 spleens at autopsy. The principles of organ sizing (namely dimensional analysis and similarity theory) are applied to derive a number of equations, a nomogram, and convenient spleen size indices for assessing both the absolute spleen weight and the allometric size of the spleen (namely the size of the spleen relative to the size of the subject) in health and disease. It is shown that occult splenomegaly can be diagnosed with confidence in nuclear medicine before spleen weight is increased 1.3‐fold (on average); in radiology, if the whole spleen projection is visible, splenomegaly can be diagnosed at that value. If only the tip of the spleen is visible as in most plain abdominal radiographs then latent splenomegaly can be diagnosed when, on average, spleen weight is increased 2‐fold. Palpable splenomegaly usually corresponds to a 3.4‐fold increase in spleen weight above the normal value in health.