Influence of collection site on cerebrospinal fluid test results in horses with equine protozoal myeloencephalitis

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BACKGROUND: Cerebrospinal fluid (CSF) analysis is a critical component in the diagnosis of equine protozoal myeloencephalitis (EPM). Test results obtained may differ based on different collection sites. HYPOTHESIS/OBJECTIVES: Compare results of EPM-specific diagnostic tests obtained from different collections sites in EPM-affected and non-EPM affected horses. ANIMALS: Twenty control and 7 EPM-affected horses. METHODS: Prospective observational study. CSF was collected from the lumbosacral (LS) and cranial sites (atlantooccipital or C1-2) in 27 horses and EPM-specific diagnostic tests were performed. Data were summarized by collection site and disease status. Results were compared by Wilcoxon signed rank test in the non-EPM and EPM-affected cohorts. RESULTS: The EPM-specific diagnostic test results (anti-Sarcocystis neurona CSF antibody titer, serum/CSF titer ratios, and S. neurona antibody index [AI]) in non-EPM affected horses did not differ based on collection site. In EPM-affected horses the median [interquartile range] CSF anti-S. neurona antibody titers were higher in samples from the LS site compared with fluid collected from the cranial sites (160 [1240] vs. 80 [600]; Wald test statistic, 21.0; P = .03) and the median serum/CSF titer ratio was also lower in fluid collected from the LS site (25 [46.8] vs 50 [93.7]; Wald test statistic, 0; P = .03). Median AI values did not differ based on CSF collection site. CONCLUSIONS AND CLINICAL IMPORTANCE: Results of EPM diagnostic tests on CSF may differ based on collection site and lead to misdiagnosis. The AI appears to be less affected by collection site than antibody titers or the serum/CSF titer ratio.

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