The predominance and diagnostic value of neutrophils in differentiating transudates and exudates in dogs.

BACKGROUND: There is disagreement in the literature about the proportion of neutrophils expected in canine transudates. A cutoff of <30% neutrophils has been recommended for distinguishing transudates from exudates, but its validity has not been established.

OBJECTIVE: The aim of this study was to evaluate differential cell counts in canine effusions and analyze the percentage and number of neutrophils in transudates and exudates.

METHODS: Effusion data were obtained retrospectively from 263 dogs with pleural or peritoneal effusion. Low-protein transudates, high-protein transudates, and exudates were classified using the total protein (TP) concentration and total nucleated cell count (TNCC). Differential percentages and absolute neutrophil counts were compared by the effusion type and underlying etiology.

RESULTS: Low-protein transudates (n = 63), high-protein transudates (n = 84), and exudates (n = 77) had a median (range) of 35% (0%-100%), 59% (0%-100%), and 90% (50%-98%) neutrophils (P < .0001). All effusions with <50% neutrophils were transudates, but 53% of transudates had ≥50% neutrophils, and 69% had ≥30%. Median neutrophil counts were 62/µL (0-892/µL), 538/µL (0-4550/µL), and 45 590/µL (5400-496 800/µL) in low-protein transudates, high-protein transudates, and exudates, respectively (P < .0001). Neutrophil counts correlated with TNCC (r2  = 0.99), such that using neutrophil cutoffs did not affect effusion classifications in most cases. Neutrophil percentages and counts were higher in effusions from dogs with uroabdomen and sepsis (P < .01); neutrophil counts were lower in dogs with hepatic insufficiency (P < .0001). Uroabdomen usually caused low-protein, high-neutrophil exudates.

CONCLUSIONS: Although effusions with <50% neutrophils are transudates, most transudates and exudates have ≥50% neutrophils, limiting the diagnostic usefulness of % neutrophils for classifying effusions. Absolute neutrophil cutoffs did not notably improve effusion classification but could warrant future studies.