Use of albumin quotient and IgG index to differentiate blood‐ vs brain‐derived proteins in the cerebrospinal fluid of cats with feline infectious peritonitis

Steinberg, Tanja Alexandra; Boettcher, Irene Christine; Matiasek, Kaspar; Hirschvogel, Katrin; Hartmann, Katrin; Fischer, Andrea; Kuntz, Anne

Background: Inflammation of the central nervous system (CNS) is a frequent condition in cats but etiology often remains unsolved. Routine cerebrospinal fluid (CSF) analysis can be extended through the calculation of the albumin quotient (Q (alb)), a marker of the integrity of the blood–brain barrier (BBB), and IgG index, an estimate of intrathecal IgG synthesis. Objectives: The purpose of this study was to validate nephelometric methods for CSF protein analysis, and to use the Q (alb) and IgG index to discriminate blood‐ and brain‐derived immunoglobulin fractions in cats with feline infectious peritonitis (FIP). Methods: Cats presented to our clinic between 2001 and 2005 were included in the study based on clinical and laboratory data and histopathologic findings at necropsy. Cats were grouped as having nonneurologic disease (controls; n=37), brain tumors (n=8), FIP involving the CNS (n=12), and extraneural FIP (n=12). CSF‐total protein (TP) was measured and albumin and IgG concentrations were measured in paired CSF/serum samples; Q (alb) and IgG index were calculated. Intraassay and interassay precision of the nephelometric assays were determined using pooled samples. Results: Coefficients of variation for the nephelometric assays ranged from 2.7% to 7.2%. In control cats, CSF‐TP concentration ranged from 0.06 to 0.36 g/L, Q (alb) ranged from 0.6 to 5.7 × 10(−3), and IgG index ranged from 0.3 to 0.6. Q (alb) and IgG index were significantly higher in cats with brain tumors and cats with CNS‐FIP compared with other groups. Compared with control cats, pleocytosis was evident in 8 of 12 (67%) cats and CSF‐TP was increased in 3 of 12 (25%) cats with CNS‐FIP. Conclusion: Nephelometry is a reliable method for measurement of CSF protein, albumin, and IgG in cats. The Q (alb) and IgG index did not identify a CSF protein pattern specific for BBB dysfunction or intrathecal IgG synthesis in cats with CNS‐FIP.