Is it necessary to compare the daily readings?

No. The control line (C) shows the same intensity of color for each test, so that each test can be interpreted independently. Each daily reading tells you whether or not you are about to ovulate. If the test result line (T) shows a similar or darker color than the control line (C) you have detected your LH surge. Ovulation should occur within 24-36 hours after the first positive result.