Effects of blood contamination of cerebrospinal fluid on western blot analysis for detection of antibodies against Sarcocystis neurona and on albuminquotient and immunoglobulin G index in horses

Objective - To determine effects of blood contamination on western blot (VV B) analysis of CSF samples for detection of anti-Sarcocystis neurona antibo dies, and on CSF albumin and IgG concentrations, albumin quotient (AQ), and IgG index in horses. Design - Prospective in vitro study. Samples - Blood with various degrees of immunoreactivity against S neurona was collected from 12 healthy horses. Cerebrospinal fluid without immunorea ctivity against S neurona was harvested from 4 recently euthanatized horses . Procedure - Blood was serially diluted with pooled nonimmunoreactive CSF so that final dilutions corresponded to 10(-3) to 100 mu l of blood/ml CSF, a nd WB analysis was performed on contaminated CSF samples. Number of RBC, al bumin and IgG concentrations, AQ, and IgG index were also determined. Results - Antibodies against S neurona were detected in CSF contaminated wi th 10(-3) mu l of strongly immunoreactive blood/ml. In CSF samples contamin ated with 10 Cll of blood/ml, AQ remained within reference range. Volume of blood required to increase IgG index varied among blood samples and was pr imarily influenced by serum IgG concentrations. Number of RBC in contaminat ed samples was correlated with volume of blood added, but not with degree o f immunoreactivity detected in contaminated CSF samples. Conclusions and Clinical Relevance - During collection of CSF from horses, contamination with blood may introduce serum antibodies against S neurona a t concentrations sufficient for detection by WE analysis, thus yielding fal se-positive results. When blood is moderately or strongly immunoreactive, t he amount of contaminating albumin may be small enough as to not increase A Q above reference range. In these cases, AQ and IgG index should be interpr eted with caution.