Clean sport competition is of significant concern to many governments and sporting organizations. Highly sensitive and rapid sensors are needed to improve the detection of performance enhancing drugs in sports as athletes take diuretics to dilute the concentration of drugs in their urine and microdose under the detectable limits of current sensors. Here we demonstrate, using frequency locked microtoroid optical resonators, a 3 orders of magnitude improvement in detection limit over the current gold standard, mass spectrometry, for the common performance enhancing drug, human chorionic gonadotropin (hCG). hCG, also known as the pregnancy hormone, was detected both in simulated urine and in the urine of pregnant donors at a concentration of 1 and 3 femtomolar, respectively. We anticipate that the sensitivity provided by frequency locked optical microcavities can enable a new standard in antidoping research.