An LH strip is positive when the test line is as dark as or darker than the control line. That’s the single most important rule, and it trips up a lot of people because unlike a pregnancy test, a faint line on an LH strip does not mean a positive result. Your body always produces some luteinizing hormone, so the test line will almost always show at least a faint mark. What you’re looking for is a surge, the moment that line matches or exceeds the control in intensity.
What the Two Lines Mean
Every LH strip has two lines: a control line (C) and a test line (T). The control line appears every time the strip works correctly. If no control line shows up, the strip is faulty and you should discard it and test again with a new one.
The test line reflects the concentration of luteinizing hormone in your urine. Here’s how to interpret it:
- No test line or very faint test line: LH levels are low. You are not near your surge.
- Test line lighter than the control: Still negative. LH is present but hasn’t surged yet. This is the result that confuses most people. A visible but lighter line means “not yet.”
- Test line equal to or darker than the control: Positive. Your LH is surging, and ovulation is likely within 12 to 48 hours.
Color comparison can be tricky under artificial lighting. Read your strip in natural light, and read it at the time window specified in the instructions, usually around five minutes. Reading too early or too late can distort the result.
How the Surge Relates to Ovulation
A positive LH strip doesn’t mean you’re ovulating right now. It means you’re about to. LH surges roughly 24 to 48 hours before the egg is released, and ovulation itself typically happens 8 to 20 hours after LH hits its absolute peak. Since a strip can’t tell you the exact moment of the peak (only that you’ve crossed a threshold), the practical window is this: once you get a positive, your most fertile time is the next one to two days.
Most strips use a detection threshold between 25 and 30 mIU/mL, which research has shown offers the best balance of accuracy and timing. Some brands use lower thresholds (20 mIU/mL) and may pick up the surge earlier, while others use higher ones (35 or 40 mIU/mL) and may catch it slightly later. Knowing your brand’s threshold can help you understand why results might differ between products.
When and How to Test
The best time to test is with your second morning urine, roughly between 10 a.m. and noon. First morning urine can sometimes give misleading results because LH is synthesized in pulses and may not have accumulated in the same way overnight. Some people prefer testing twice a day, once in the late morning and again in the early evening, to avoid missing a short surge.
Reduce your fluid intake for about four hours before testing. Drinking a lot of water dilutes your urine and can make a real surge look like a negative. You don’t need to dehydrate yourself, just avoid chugging water right before you test. Dip the strip in a cup of urine up to the marked line, wait the specified time (usually five minutes), then read it.
When to Start Testing Each Cycle
If your cycle is 28 days, start testing around day 10 or 11. For shorter cycles, start earlier; for longer or irregular cycles, start a few days after your period ends and test daily until you see the surge. You’ll typically need to test for at least 8 days per cycle to reliably catch it, though some people find their surge in fewer days once they know their pattern.
Using an App to Read Your Strips
Smartphone apps can remove some of the guesswork from reading line intensity. These apps photograph your strip and calculate a T/C ratio, comparing the color intensity of the test line to the control line numerically. A ratio below 1.0 means the test line is lighter (negative). A ratio at or above 1.0 means it matches or exceeds the control (positive).
The real advantage of app-based tracking isn’t the single reading. It’s the chart it builds over time. By logging daily results, the app shows you how your LH levels rise and fall across your cycle, making it easier to spot your personal surge pattern rather than relying on a single snapshot. This is especially useful if your surges are subtle or your lines are hard to compare visually.
Not Every Surge Looks the Same
Many people expect a single dramatic spike: negative, positive, negative. But LH surges vary considerably from person to person and even cycle to cycle. Some women experience a sharp, obvious peak that lasts less than a day. Others have a broader plateau where the test line stays dark for two or three days. Research has shown that surges with multiple peaks or those lasting more than three days are associated with slightly different hormonal profiles, but they still represent normal, fertile cycles.
This is why tracking over multiple cycles matters more than any single test. Your pattern might not match the textbook example, but if it’s consistent for you, it’s still useful information. Compare your results to your own history, not to an idealized curve.
Why Some People Get Misleading Results
The most common reason for confusing results is simply reading a faint line as positive. But there are genuine biological reasons strips can mislead you, and the biggest one is polycystic ovary syndrome (PCOS).
Women with PCOS often have baseline LH levels that are significantly higher than average. One study found that women with PCOS had an average LH level of about 12 mIU/mL outside of ovulation, compared to roughly 2.4 mIU/mL in women without the condition. Since most strips trigger a positive at 25 to 30 mIU/mL, that elevated baseline means LH doesn’t need to rise as much to cross the threshold. The result can be frequent false positives, where the strip reads positive but no egg is actually being released. In some cases, PCOS causes cycles without ovulation at all, yet the strips still show positive results.
If you have PCOS or irregular cycles and find that you’re getting multiple positives per cycle (or positives that don’t seem to line up with ovulation), LH strips alone may not be reliable for you. Combining them with other signs of fertility, like tracking cervical mucus, significantly improves accuracy. Research has found that pairing a positive LH test at the 25 mIU/mL threshold with peak-type cervical mucus raises the specificity of ovulation prediction to 97 to 99 percent, compared to 91 percent for LH testing alone.
Common Mistakes That Affect Accuracy
Beyond medical conditions, a few practical errors account for most bad readings. Testing with very dilute urine is the most frequent one, and it’s easily fixed by limiting fluids beforehand. Reading the strip outside the recommended time window is another: dye can continue migrating on the strip after the reading period, making a negative look falsely darker over time. Never go back and re-read a strip hours later.
Using expired strips, storing them in humid environments like a bathroom cabinet, or exposing them to extreme heat can all degrade the antibodies on the strip and produce unreliable results. If you buy strips in bulk, store them sealed in a cool, dry place and check the expiration date before each cycle.