Purpose of the study: Leukocyte esterase is an enzyme in neutrophils from which it is released into exudate; its detection by colorimetric test strips indicates the presence of neutrophils. This is a rapid method to find whether exudate is of infectious or non-infectious aetiology. The aim of the study was to determine the sensitivity and specificity of leukocyte esterase testing with use of AUTION Sticks (Arkray) for examination of exudates obtained in inflammatory diseases of the skeletal system.
Material and methods: Exudates associated with skeletal system diseases were collected from 45 patients in the period from July 1st to December 31 st , 2012. Aspirates obtained under sterile conditions were examined for leukocyte esterase; cytological and microbiological examinations were also carried out. For the detection of leukocyte esterase, a drop of aspirate was placed on the reagent zone of a test strip and the resulting colour reaction was read after 90 minutes. Changes in colour were compared with a reference strip provided by the manufacturer. The results were assessed on a five-shade scale as follows: 0, no colour change; 1 to 4, gradual change from light pink to deep purple. The results were compared with those of cytological and microbiological examinations. Shade 4 on the strip corresponded to a positive cytological finding of bacterial infection, and shades 3 and 4 correlated with a positive microbial finding. The sensitivity and specificity of leukocyte esterase testing were statistically evaluated for both comparisons.
Results: Based on the results of cytological and microbiological examinations, an infectious aetiology of exudate was diagnosed in 21 (44.4%) and non-infectious aetiology in 24 (63.6%) patients. With leukocyte esterase reagent strips when shade 4 was taken as a positive result, the sensitivity and specificity of examination was assessed as 0.6190 and 0.9583, respectively. When taking both shade 3 and shade 4 for a positive result, sensitivity and specificity were 0.8571 and 0.8750, respectively. Shades 0 and 1 corresponded to the number of leukocytes in exudate that was no higher than 2 x 10⁹/ml.
Discussion: The detection of leukocyte esterase is a quick and easy examination. It is useful for readily excluding or confirming an infectious aetiology of exudate and can, to some extent, substitute a cytological examination. It can also help to make a quick decision whether one- or two-stage joint reimplantation should be performed and thus eliminate the need of intra-operative histological examination of frozen tissue samples. A drawback of the method was that exudate samples contaminated with blood interfered with an assessment of colour shades. However, this can be avoided by centrifugation of the sample and use of a supernatant free from erythrocytes.
Conclusions: Diagnosing infectious aetiology of joint exudate or exudate from an abscess using leukocyte esterase reagent strips appears, according to our results, to be a promising, semi-quantitative method with high specificity and sensitivity which is rapid, simple and affordable. It can be useful particularly in out-patient institutions for a quick diagnosis of arthritis; intraoperatively, it can serve as an additional method to other exudate examinations.