The relationship between parent compounds and metabolites has been investigated further (figure ), including calculating ratios for individual sampling points for the two catchment areas. Applying correlation statistics between AAs and associated metabolites showed mostly strong correlations in mass loads for both Bath and Keynsham (table ). CLR and its major metabolite dmCLR demonstrated the strongest correlations (r=0.9, p≤0.05 and r=0.8, p≤0.05 for Bath and Keynsham, respectively). Furthermore, correlations between AAs that are prescribed as a combination were also observed. For example, SMX and TMP are commonly prescribed together in the UK as a combination known as cotrimoxazole (National Institute for Health and Care Excellence, n.d.). This is due to the different mechanisms of action and combined ability to tackle infections to reduce emerging AA resistance. Positive correlations were observed between SMX and TMP (r value 0.5 and 0.6 for Bath and Keynsham respectively, both p values ≤0.05), aSMX and TMP (r value 0.6 for both Bath and Keynsham, p values ≤0.05). Furthermore, co-trimoxazole is one-part TMP to five parts SMX. On average, whilst loads of SMX in wastewater were higher in general than TMP, the ratio between loads of SMX:TMP were 1.41 ± 0.50 and 2.83 ± 2.41 for Bath and Keynsham, respectively. Reasoning why the ratio between the two is lower than expected could be due to differing metabolism patterns of SMX and TMP, and that TMP can be prescribed by itself. The comparison of ratios between AA/metabolites in the two sites was investigated (figure ). Due to the presence of outliers, medians were also calculated. In general, median values were close to that of the average, demonstrating normal distribution (table ). The ratio of CLR/dmCLR in Bath and Keynsham had concordant results, of 2.74 ± 0.63 and 2.81 ± 0.90 respectively. This is potentially attributed to both CLR and dmCLR having demonstrated good stability in wastewater, with minimal degradation over a 24-hour period at room temperature . The SMX/aSMX ratio, again, was consistent across both catchment areas at 2.40 ± 1.23 and 2.52 ± 1.96 for Bath and Keynsham. During June and July 2019, in Keynsham, several days measured ratios of ~10. These days often had lower levels of aSMX in wastewater, whereas no noticeable increase of SMX. SMX is taken orally in the UK, via tablet or liquid suspension and no topical usages have been identified (National Institute for Health and Care Excellence, n.d.). The lower metabolite levels in Keynsham could potentially be due to direct disposal of the parent pharmaceutical into the sewage system.