Feline Infectious Peritonitis (FIP) is caused by a systemic feline coronavirus (FCoV). Prior to June 2024, compounded FIP treatment was unavailable for prescription by veterinarians in the United States, leading to many cat owners obtaining treatment through unlicensed “black market” sources. We hypothesized that clinicopathologic data could provide insight on prognostic indicators for the treatment of FIP with GS-441524. This study used data gathered via surveys from 126 cat owners who used “black market” GS-441524 for their cats. We compared bloodwork parameters over twelve weeks of treatment. None of the clinicopathologic correlates, when analyzed via two-sample t-tests, produced statistically significant results between cured, deceased, and relapsed groups. Within cats considered cured, it was observed that hematocrit (HCT) and white blood cell (WBC) values were within normal limits by the 2–6-week period. Cats who died during the study had lower HCT and higher WBC values within the 2–6-week period. Trends were also seen in A/G and total bilirubin (T-BIL), with deceased patients showing a higher A/G ratio and lower value than those in the cured group. Overall, these data demonstrate a lack of traditional clinicopathologic parameters which are consistently predictive of FIP therapy success. Other predictors of outcome with antiviral therapy should be pursued.