Predictive values, sensitivity and specificity of abdominal fluid variables in determining the need for surgery in horses with an acute abdominal crisis

Objective To determine the predictive values, sensitivity and specificity of abdominal fluid variables associated with the need for surgery in horses with an acute abdominal crisis. Design Retrospective study. Animals Two‐hundred and thirty‐six horses examined for signs of abdominal pain between January 1993 and June 1999. Methods Breed, age and gender of the horse and colour, total protein concentration and total nucleated cell count of an abdominal fluid sample were recorded. Colour of the abdominal fluid was classified as normal if it was yellow and transparent. Turbid fluid or fluid that was serosanguinous or other colours was classified as abnormal. Protein concentration ≤ 20 g/L and a total nucleated cell count ≤ 5 times 10 9 cells/L were considered normal and values above these were considered abnormal. An abdominal fluid sample was classified as abnormal if one or more of the three variables were considered abnormal. Cases were defined as surgical when lesions identified at surgery or necropsy examination would not have resolved with medical treatment alone. Cases were defined as medical in horses that survived without surgical intervention, and those with a lesion found at surgery or necropsy that would have resolved with medical treatment alone. A third category was identified during the study as those diagnosed with Actinobacillus equuli induced peritonitis. These horses were included in the study but not in the data analysis. Data analysis The association between the sensitivity, specificity and positive and negative predictive value of colour, total protein, and total nucleated cell count in the abdominal fluid and the need for surgery was calculated. Results There were 100 females and 136 males of mixed breeds, ranging from 3 days to 26 years of age that had an abdominocentesis performed during the specified period. There were 97 horses with a lesion classified as surgical, 91 horses with a lesion classified as medical and 48 horses with a diagnosis of A equuli induced peritonitis. Colour of the abdominal fluid was recorded in all horses, protein concentration was recorded in 194 horses and total nucleated cell count was recorded in 179 horses. Abnormal abdominal fluid colour had a sensitivity, specificity, positive and negative predictive value of 92%, 74%, 79% and 89% respectively, associated with the need for surgery. Sensitivity, specificity, positive and negative predictive values for a serosanguinous abdominal fluid sample associated with the need for surgery were 48%, 99%, 98% and 64% respectively. Abnormal abdominal fluid protein concentration had a sensitivity, specificity, positive and negative predictive value of 86%, 75%, 77% and 85% respectively, associated with the need for surgery. The sensitivity, specificity, positive and negative predictive value associated with the need for surgery in horses with an abnormal total nucleated cell count in the abdominal fluid were 59%, 75%, 67% and 67%, respectively. An abdominal fluid sample classified as abnormal had a sensitivity, specificity, positive and negative predictive value of 92%, 74%, 79% and 89% respectively, associated with the need for surgery. Conclusion Results of this study suggest that abdominal fluid sample analysis contributes to the decision to proceed to surgery, but is not a diagnostic panacea. Colour and protein concentration of abdominal fluid were the most useful variables in abdominal fluid for differentiating medical and surgical lesions. Colour and protein had a greater value in horses with a disease likely to respond to medical treatment (negative predictive value) than those with a lesion requiring surgery (positive predictive value) except when the fluid was serosanguinous. Abdominal fluid colour and protein are clinically relevant and easily measured in the field, providing immediate information without the need for sophisticated laboratory techniques.