Ovulation predictor kits are used to measure the amount of luteinizing hormone (LH) that is filtered from the blood into the urine. A positive LH test detects this increase, indicating that ovulation is about to take place. This test does not, however, confirm that ovulation occurred. It merely demonstrates that hormonal circumstances are right for it to occur. This test is most useful for timing intercourse and intrauterine insemination. During the testing phase of the fertility evaluation, the LH kit is used to time the progesterone blood draw (7-8 days after the LH surge) or during the evaluation for recurrent pregnancy loss, the timing for the endometrial biopsy (12-13 days after the surge).
LH testing is most reliable for women whose menstral cycles occur between 24-35 days apart. Cycles shorter than 22 days or longer than 38 days apart tend to be anovulatory. Some women with anovulatory cycles who have PCOS have chronically elevated LH levels which leads to a false positive test result.
Ovulation usually occurs 14 days prior to the NEXT menses. For women who have monthly menstral cycles, subtract 16 days from the shortest cycle and 12 days from the longest cycle to establish the fertile window. Testing should begin at 16 days prior to the shortest cycle and continue until the control line and test line are of equal darkness or the test line becomes darker than the control line. It is not helpful to continue testing after that point. Ovulation occurs 36 hours after the peak LH which corresponds to between 18-42 hours after a positive LH test.
When using the LH test kit, it is best to allow the urine to be somewhat concentrated. If testing in the late afternoon, hold off drinking large quantities of fluids after lunch until performing the test.