GS-441524 sample analysis: estimating concentrations

The complete write-up of the results and methods for the GS-441524 sample analysis can be found here

In addition to analytical chemistry results, it is important to also collect data to better understand treatment outcomes. I suggest using the FIPLog website that I created. This website helps folks track and monitor their injections and observations during the 84-day treatment period.


We tested 15 different samples of GS. This included several popular brands in current use, several older brands suspected of having issues, and several very new brands that do not have substantial clinical testing. 

We compared each sample’s concentration and relative purity against a reference sample of known concentration.  Results suggest that some brands are likely to have higher concentrations than advertised, while other brands have concentrations that may not meet the marketed concentration. 

Summary Data

The following summary table can be used to help guide brand selection and dose estimation for individuals seeking to treat their cats for FIP. 

Brands where multiple batches were tested appear to show higher variability than brands where only one batch was tested. Concentration estimates in the table show the 95% confidence interval for estimating the true concentration of a given sample.  Brands with concentration estimates that fall outside of the marketed concentration at the low-end should be used with caution (increasing suggested doses), or avoided until additional testing can be completed.

BrandMeasured Concentration (95% CI)Marketed Concentration (mg/mL)Likely to be in Spec?# of Samples Tested
Ajax16.9 – 17.915Yes1
BadShire7.4 – 8.915No1
BEAT15.3 – 18.415Yes1
Blossom6.3 – 815No1
Brava17.3 – 19.615Yes1
Capella16.3 – 19.415Yes2
Hero_White_Cap13.8 – 15.516.5No1
Mutian15.6 – 17.316.7Maybe1
Pine_green13.4 – 15.815Maybe1
Pine_red11 – 14.915No1
Ruby15.1 – 18.515Yes1
Shire13.4 – 14.615No1
Spark_blue18.3 – 26.420Maybe1
Spark_purple13.6 – 17.715Maybe3
Spark_red20.3 – 26.417Yes1
These test results should not be used as the sole determinant for choosing a GS brand. It is important to also consider the treatment outcomes for brands based on well-established clinical successes for a large number of cats, both through the 84-day treatment and observation periods. There may be other factors, including sample contamination, that cannot be detected by the UV-vis methodology employed in this study.
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