Utility of the Ratio between Lactate Dehydrogenase (LDH) Activity and Total Nucleated Cell Counts in Effusions (LDH/TNCC Ratio) for the Diagnosis of Feline Infectious Peritonitis (FIP)

SIMPLE SUMMARY: The concentration of lactate dehydrogenase (LDH) in the cavitary effusions from cats with feline infectious peritonitis (FIP) is high, as in other types of effusions, but the total nucleated cell count (TNCC) of the fluid is typically low in this disease. Therefore, we hypothesized that the LDH/TNCC ratio is higher in effusions from cats with FIP than in other effusions. To assess this hypothesis, the LDH/TNCC ratio recorded in 648 effusion of different types were classified based on the probability of FIP estimated through additional tests on fluids, blood or tissues. This approach confirmed that this ratio is higher in FIP effusions or with high probability of FIP. We provide some practical recommendation: when the laboratory receives and analyzes only the fluid, an LDH/TNCC ratio higher than 0.72 maximizes the possibility to correctly diagnose FIP (sensitivity and specificity of 79%). A ratio higher than 7.54 indicates a 6-fold higher probability to have FIP than another disease. Lower thresholds (0.54 and 2.27) have higher sensitivity and specificity (around 82%) respectively, or likelihood to have FIP (more than 10 times) when other changes consistent with FIP in blood or tissues are detected. ABSTRACT: Background: We tested the hypothesis that the ratio between lactate dehydrogenase activity (LDH) and total nucleated cell counts (TNCC) in effusions may be useful to diagnose feline infectious peritonitis (FIP). Methods: LDH/TNCC ratio was retrospectively evaluated in 648 effusions grouped based on cytology and physicochemical analysis (step 1), on the probability of FIP estimated by additional tests on fluids (step 2) or on other biological samples (step 3, n = 471). Results of different steps were statistically compared. Receiver Operating Characteristic (ROC) curves were designed to assess whether the ratio identify the samples with FIP “probable/almost confirmed”. The cut-offs with the highest positive likelihood ratio (LR+) or Youden Index (YI) or with equal sensitivity and specificity were determined. Results: A high median LDH/TNCC ratio was found in FIP effusions (step1: 2.01) and with probable or almost confirmed FIP (step 2: 1.99; 2.20 respectively; step 3: 1.26; 2.30 respectively). The optimal cut-offs were 7.54 (LR+ 6.58), 0.62 (IY 0.67, sensitivity: 89.1%; specificity 77.7%), 0.72 (sensitivity and specificity: 79.2%) in step 2 and 2.27 (LR+ 10.39), 0.62 (IY 0.65, sensitivity: 82.1%; specificity 83.0%), 0.54 (sensitivity: 82.1%; specificity 81.9%) in step 3. Conclusions: a high LDH/TNCC ratio support a FIP diagnosis.