Original articleUsefulness and limitations of rapid urine dipstick testing for joint-fluid analysis. Prospective single-center study of 98 specimens
Introduction
Joint fluid analysis is pivotal in the diagnostic workup for joint effusions and arthritis. A high leukocyte count indicates an inflammatory process. The most widely used cut-off is 2000/mm3, despite some uncertainty about this choice [1], [2].
Septic arthritis is defined as the presence of culturable bacteria in the joint fluid and metabolic arthritis as the presence of microcrystals visible by polarized light microscopy (sodium urate in gout and calcium pyrophosphate dihydrate in articular chondrocalcinosis). No specific joint-fluid criteria exist for diagnosing inflammatory joint disease.
Reagent strip testing of urine is a validated tool for the rapid diagnosis of urinary tract infections via the detection of leukocyte esterase activity and/or nitrites [3], [4]. Reagent strips have also been of some usefulness in analyzing other body fluids. Thus, the negative predictive value (NPV) was excellent for infection of ascitic fluid [5] and the positive predictive value (PPV) was high in bacterial meningitis [6].
Here, our objective was to evaluate the diagnostic performance of leukocyte esterase reagent strip testing (LERST) of joint fluid in separating inflammatory from mechanical disease.
Section snippets
Methods
We studied joint fluid specimens collected for diagnostic purposes from patients evaluated at a rheumatology department over a 12-month period. For LERST, the specimens were collected in dry tubes and tested within 1 hour after collection using Combur10 Test® UX strips (F. Hoffmann-La Roche AG, Basel, Switzerland). In addition to leukocyte esterase activity, these strips assess nine other physicochemical parameters: specific gravity, pH, nitrites, protein, glucose, ketone, urobilinogen,
Statistics
Sensitivity (Se), specificity (Sp), PPV, NPV, and the positive and negative likelihood ratios (LR+ and LR−) were determined using the cell count performed at the laboratory as the reference standard. Means were compared using the Kruskal-Wallis test and percentages using the chi-square test (or Fisher's exact test when sample size was less than 5). Cohen's kappa coefficient was computed to assess reproducibility. P values smaller than 0.05 were considered significant.
Results
During the study period, 98 joint fluid specimens were analyzed. The main source joints were the knee (76.5%), hip (7.1%), shoulder (6.1%), wrist (4.1%), and elbow (3.1%). Cell counts indicated mechanical disease in 26 cases and inflammatory disease in 72 cases (metabolic disease, n = 30; chronic inflammatory joint disease, n = 30; septic arthritis, n = 7; and undetermined, n = 5). Table 1 reports the mean cell counts and percentage of neutrophils in each diagnostic subgroup.
In the group of
Discussion
Our findings demonstrate that rapid reagent strip testing for leukocyte esterase activity is useful as a screening tool for identifying inflammatory joint fluids. Thus, a positive LERST had an excellent PPV of more than 95% and an LR+ greater than 10.
These results are consistent with those reported by Ravaud et al. [7], who performed what is to our knowledge the only published study evaluating LERST of joint fluid from native joints. Blood cell counts and strips were used to test 208 joint
Disclosure of interest
The authors declare that they have no conflicts of interest concerning this article.
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