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

The antibody index is the most reliable EPM diagnostic metric, as it is less affected by CSF collection site than titers or serum/CSF ratios.

This prospective observational study investigated whether cerebrospinal fluid (CSF) collection site influences EPM-specific diagnostic test results in horses. CSF was collected from both lumbosacral (LS) and cranial sites (atlantooccipital or C1-2) in 20 control horses and 7 EPM-affected horses, with EPM-specific tests including anti-Sarcocystis neurona CSF antibody titers, serum/CSF titer ratios, and the S. neurona antibody index (AI) performed on all samples. In non-EPM-affected horses, no significant differences in test results were observed between collection sites. However, in EPM-affected horses, CSF anti-S. neurona antibody titers were significantly higher in LS-collected samples compared to cranial site samples (median 160 vs. 80; P = .03), and the serum/CSF titer ratio was significantly lower in LS fluid (median 25 vs. 50; P = .03). These site-dependent differences in antibody titers and serum/CSF ratios have the potential to affect diagnostic accuracy and may lead to misclassification of EPM status depending on where CSF is collected. Notably, the antibody index (AI) did not differ significantly between collection sites in either cohort, suggesting it may be a more robust and site-independent diagnostic parameter. The authors conclude that collection site should be considered when interpreting EPM diagnostic test results and that the AI may be the preferred metric due to its relative insensitivity to collection location. Clinicians relying on antibody titers or serum/CSF ratios alone should exercise caution when CSF is collected from sites other than those used during initial diagnosis or test validation.

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