Creatinine is a waste product the human body makes from normal muscle breakdown and is filtered through the kidneys and excreted in urine. This blood test measures your level of creatinine which can help show how well your kidneys are functioning. Though very unlikely, one of the side effects of taking PrEP is having slightly increased levels of creatinine. To be on the safe side, creatinine levels are measured routinely while taking PrEP.