The use of 10% buffered formalin as a preservative agent when cerebrospinal fluid analysis is delayed.

Objectives: To evaluate the utility of 10% buffered formalin in preserving canine cerebrospinal fluid samples when analysis was delayed.

Methods: Inclusion criteria were dogs >10 kg having cerebrospinal fluid analysis performed as part of investigations at a referral hospital. Samples were submitted to an external laboratory in tubes containing Ethylenediaminetetraacetic acid (ETDA) as paired 0.5 mL samples; one with the addition of 0.05 mL of 10% buffered formalin and the other without. The samples were reviewed by a single pathologist who was blinded as to which sample contained formalin. Nucleated cell preservation was graded by the pathologist from 1 to 4 (1 being poor and 4 being excellent). Total protein was measured in both samples.

Results: Forty-five paired samples were included. There was no significant difference in detectability of nucleated cells between plain and formalin samples. Grade 3 was taken as the cut off for acceptable cell preservation. Based on all available samples and assessing the preservation of both nucleated cells and red blood cells, samples containing formalin were significantly more likely to be acceptably preserved. This preservation analysis was repeated on the 17 samples with at least one nucleated cell in both formalin and plain samples and was not statistically significant.

Clinical Significance: The addition of formalin did not significantly improve the preservation of cerebrospinal fluid samples when analysis was delayed; however, concerns raised by previous authors regarding reduced cell preservation with addition of formalin were also not confirmed. Further large-scale studies are required to investigate the effect on nucleated cell preservation.