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Ascitic fluid samples are frequently sent to the laboratory for analysis. Although the underlying cause of the ascites is often thought to be clinically obvious, it is important to establish a definitive diagnosis. The value of a cell count and bacterial culture of the ascitic fluid is not disputed, but the role of biochemical testing is less clear. The use of ascitic fluid total protein to try to classify ascitic fluids as either an exudate or a transudate has contributed to this. The use of the physiologically based serum ascites albumin gradient to differentiate ascites caused by portal hypertension from other causes provides a better diagnostic approach. We recommend that the serum ascites albumin gradient is performed by laboratories as the first-line test and that interpretative reports are provided. Additional testing should be restricted to specific diagnostic queries and requires close collaboration between the laboratory and the clinician.

IgG and IgM class insulin autoantibodies were measured by an enzyme-linked immunosorbent assay in sera from members of the Barts-Windsor-Middlesex prospective family study for Type 1 (insulin-dependent) diabetes. One hundred and twelve individuals from 28 families were selected for study on the basis of a clearly defined islet cell antibody status. IgG insulin autoantibodies were found to be significantly associated with islet cell antibody positive (n = 30) versus islet cell antibody negative (n = 57) first degree family relatives (p = 0.002), with increased significance (p = 0.0003) if complement-fixing (CF)-islet cell antibody individuals (n = 20) only were considered. In addition, a significant association of IgG insulin autoantibodies with subsequent development of diabetes was observed within the CF-islet cell antibody positive group (p less than 0.0003). No such associations were found for IgM insulin autoantibodies, but a higher prevalence of these autoantibodies was observed in islet cell antibody negative first degree relatives (n = 57) compared with a control group of 73 Blood Bank donors (p = 0.00007), and they were significantly associated with siblings (n = 48) rather than parents (n = 39), (p = 0.001). We conclude that the presence of IgG insulin autoantibodies and CF-islet cell antibodies confer more risk for future development of diabetes than the presence of either marker alone.

Biochemical examination of pleural fluid is usually done to try to identify the cause of a pleural effusion. The various analytes that have been suggested for this are reviewed and evaluated. Distinguishing whether the effusion is an exudate or transudate is a pragmatic first step. with further investigations dictated by the clinical features and these results. Total protein and lactate dehydrogenase were used first; Light's criteria were published in 1972 and since then additional markers including cholesterol, bilirubin and albumin gradient plus combinations of these have been proposed. Although combination testing does improve the sensitivity for diagnosis of an exudate. this is at the expense of specificity. Measurement of fluid to serum ratios appears to confer no advantage, and if a single test is required total protein performs as well as any. Additional tests may be useful in specific circumstances: pleural fluid pH may aid decisions over drainage of a parapneumonic effusion; glucose may indicate an effusion associated with rheumatoid arthritis; and adenosine deaminase may help with the diagnosis of tuberculous effusions.

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