Introduction A fingerprint offers a convenient, non-invasive sampling matrix for monitoring therapeutic drug use. Despite a multitude of potential applications, uptake of fingerprint drug testing has been stunted by the inability to determine, or control for, sample volume. The collection of reproducible fingerprints is highly challenging, where factors such as contact time, pressure and time of collection can influence the amount and composition of material collected. Here we demonstrate that internal metabolite markers in a fingerprint can be used to show that sufficient sample has been collected, and can reduce intra-donor variability. Methods Fingerprint samples (n=140) were collected from patients receiving the antibiotic isoniazid as part of their treatment, as well as from a drug-naive control group (n=50). The fingerprint samples were analysed for isoniazid (INH) and acetlyisoniaizid (AcINH), using liquid chromatography high resolution mass spectrometry (LC-MS). The dataset was analysed retrospectively for metabolites known to be present in eccrine sweat. Results INH or AcINH were detected in 89 % of the fingerprints collected from patients, and in 0 % of the control group. The metabolites lysine, ornithine, pyroglutamic acid and taurine were concurrently detected alongside INH/AcINH and were used to determine whether the fingerprint sample was sufficient for testing. Given sufficient sample volume, the fingerprint test for isoniazid use has sensitivity, specificity and accuracy of 100%. Normalisation to taurine was found to reduce intra-donor variability. Conclusion Fingerprints are a novel and non-invasive approach to medical adherence monitoring. Endogenous metabolites can be used as internal markers to demonstrate sufficient sample volume for fingerprint based testing and reduce intra-donor variability.