Clearblue Digital Ovulation Test identifies the surge in luteinizing hormone which happens 24-36 hours before ovulation, to identify your most fertile days. On the days you take a test, add your LH results into the Natural Cycles app (available with Clearblue & Natural Cycles Birth Control Starter Pack).Clearblue Digital Ovulation Test is over 99% accurate at detecting the LH surge.Do not use the LH surge result for contraceptive purposes. The Natural Cycles app uses temperature, period, and (optional) ovulation test data to determine your Green Day ('Not fertile') or Red Day ('Use protection'). REFILL KIT CONTAINS: 15 Clearblue Digital Ovulation Tests, 1 Clearblue Reader. This Refill pack is designed to continue logging Clearblue Ovulation Test results in the Natural Cycles app. Clearblue & Natural Cycles Birth Control Kit sold separately BACKED BY SCIENCE: The NC algorithm uses basal body temperature to identify fertile and non-fertile days and to confirm ovulation. Taking ovulation tests is optional and can help the NC algorithm predict your ovulation day IDENTIFY THE MOST FERTILE DAYS: Use Clearblue Digital Ovulation Tests to identify the surge in luteinizing hormone (LH). This happens 24-36 hours before ovulation to identify your 2 most fertile days CLEAR RESULTS: a flat line indicates No LH surge ('Negative') and a line with a surge indicates an LH surge ('Positive') OVER 99% ACCURATE: at detecting the LH surge DO NOT USE the LH surge result for contraceptive purposes. The Natural Cycles app uses temperature, period, and (optional) ovulation test data to determine your Green Day ('Not fertile') or Red Day ('Use protection') Directions: Testing Strips