Role of sequential leucocyte counts and C-reactive protein measurements in acute appendicitis Sir We read with interest the paper by Mr Thompson and colleagues (Br J Surg 1992; 79: 822-4) describing the use of serial leucocyte counts as an adjunct to clinical assessment in the diagnosis of acute appendicitis. The test described (that ofan increasein the total leucocyte count over a 4-h period) was shown to have a sensitivity of 92 per cent and a specificity of 100 per cent and was therefore claimed to fulfil the criteria for a diagnostic test that could prove useful in assessing patients with equivocal clinical findings.Of more practical relevance, however, is the negative predictive value of the test, the proportion of patients with a negative test result who did not have appendicitis. Although it is not quoted in the text, we calculate this value to be only 78 per cent. Furthermore, as the authors mention, those false-negative results (patients incorrectly predicted not to have appendicitis) were in cases of a perforated appendix and thus represent the very patients one would not wish to miss. Although a rising leucocyte count may be useful in certain cases, we would advocate extreme caution before refuting the diagnosis of acute appendicitis on the basis of this test.