Sampling the fluid (Pleurisy)

When fluid is present in amounts sufficient to give rise to physical signs, it is always wise to take a sample ; this is done to confirm the presence of fluid in a doubtful case and to proceed from the recognition of its presence to an attempt to discover why it is there. Sampling should always be carried out with full aseptic precautions. The best site for exploratory puncture (or thoracentesis) is decided after careful scrutiny of postero-anterior and lateral films, and the puncture is made with a needle mounted on a syringe after infiltration with a local anaesthetic right down to the pleura. In an average case the seventh or eighth interspace in the mid-axillary line is a suitable site. The patient may either be sitting up and leaning slightly forwards with the arms resting on a bed-table, or lying on his side with the site of the effusion uppermost.

The fluid is generally straw-coloured (very rarely, if trauma be excluded, haemorrhagic) and clear. Sometimes a little clot may form on standing. Lymphocytes are abundant and generally predominate, although occasionally even tuberculous fluid is distinctly polymorphonuclear in its cellular content in the earlier phases (Wihman, 1948 ; Kraft, 1949). Usually no organism can be seen and culture is sterile on all ordinary media. Inoculation into a guinea-pig may produce tuberculous lesions in up to 50 per cent of cases, and culture on Lowenstein-Jensen's (or Dubos') medium may be positive for tubercle. Increasing use will probably be made of cultural methods as techniques are improved and standardized.

Close (1946) gives some interesting figures concerning the relative merits of guinea-pig inoculation and culture in the detection of tubercle bacilli in pleural fluid, and also of the increasing number of positive results which come with ex­perience. Out of 34 tubercle-positive fluids, 10 were positive to both guinea-pig inoculation and culture, and 24 were positive to culture but negative to guinea-pig inoculation. Of 11 attempts to demonstrate tubercle bacilli in fluid, made in 1941, none was successful ; in 1945, however, 16 were positive out of 23. There is no doubt that the technique employed is of the greatest importance. More recently (Gelenger and Wiggers, 1949 ; Calnan and his colleagues, 1951) the glucose content of the fluid has been investigated. There is some doubt about the level of glucose which is critical, but low values strongly suggest a tuberculous origin whilst high levels are found in non-tuberculous conditions.