Influence of collection site on cerebrospinal fluid test n horses with EPM

DrO’s summary:
The study shows that collection site matters for some EPM tests. Titers and serum/CSF ratios can vary significantly depending on where CSF is collected. AI remains consistent and is therefore the most dependable diagnostic measure across sites.
What the AI test is
The Antibody Index (AI) determines whether S. neurona antibodies found in CSF are Produced inside the CNS (true infection), or simply leaked in from serum (false‑positive CSF titer). It does this by mathematically correcting for blood contamination and comparing specific antibody production to total immunoglobulin production.
How the AI test is performed
1. Collect paired samples
Serum sample
CSF sample (LS or AO/C1–2)
2. Measure two things in both fluids
-Specific antibody titer against S. neurona (usually by IFAT or ELISA)
-Total IgG concentration (by immunoassay)
3. Calculate the Serum/CSF ratios
𝐴𝐼 = (Serum IgG / CSF IgG) / (Serum Sn Ab titer / CSF Sn Ab titer)
4. Interpretation
Some labs use:
AI ≥ 2 as “strongly positive”
AI 1–2 as “borderline”
AI < 1 as “negative”
DrO

Influence of collection site on cerebrospinal fluid test results in horses with equine protozoal myeloencephalitis
J Vet Intern Med. 2026 Jul 1;40(4):aalag135. doi: 10.1093/jvimsj/aalag135.
Author
Martin O Furr 1
Affiliation

1 College of Veterinary Medicine, Oklahoma State University, Stillwater OK 74078, United States.

PMID: 42418837
DOI: 10.1093/jvimsj/aalag135

Abstract

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.

Keywords: Sarcocystis neurona; antibody index; cerebrospinal fluid; equine.

© The Author(s) 2026. Published by Oxford University Press on behalf of the American College of Veterinary Internal Medicine.

• This topic was modified 1 minute ago by Robert Oglesby DVM.
• This topic was modified 55 seconds ago by Robert Oglesby DVM.