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The use of urine test strips (e.g., Roche Chemstrip®) has become the standard for quickly assessing the physiological condition and/or health of wild primates. These strips have been used to detect ketosis as a marker of fat catabolism in several primate taxa in their natural environments in response to changing food availability. However, the use of urine strips to determine ketosis has only been validated in human studies, and thus it remains unclear whether these strips accurately detect and quantify ketone bodies in nonhuman primates. We examined variations in ketone body concentrations in urine samples collected from wild Bornean orangutans at the Tuanan Orangutan Research Station. We assessed the accuracy of qualitative results from Chemstrip test strips in the field (i.e., negative, small, moderate, and large) using an enzyme-linked assay in the laboratory to determine the concentrations of acetoacetate of the same urine samples. Urine samples that tested positive for ketones in the field had significantly higher levels of ketones in the enzymatic assay compared to those that tested negative. There was significant variation in acetoacetate concentrations among the 4 Chemstrip values; however, post hoc tests revealed no significant differences between negative and small samples. We conclude that urinary test strips provide a useful tool for determining ketotic state in wild orangutans, but caution should be taken when interpreting results from samples showing only small levels of ketones on these strips.