Spend any amount of time perusing TTC (trying to conceive) forums or talking with friends who are knee-deep in their own pregnancy attempts and you’ll learn that home pregnancy tests (HPTs) are fickle.

Among the things that can affect a HPT’s accuracy are:

  • evaporation lines
  • expiration dates
  • exposure to the elements
  • time of day
  • how dehydrated you are
  • color of dye (pro tip from a Healthliner: pink dye tests are better)
  • how long you waited between peeing and looking at the result
  • whether the wind speed is precisely 7 miles per hour east-southeast (OK, you got us — we’re kidding about this last one, but when you’re TTC, it can sure feel like everything matters)

Long story short: These tests are super sensitive to a variety of factors. And while they do pretty well at what they’re supposed to do — detect the pregnancy hormone human chorionic gonadotropin (hCG) — to get accurate results, you need to follow package instructions as written.

So no, you can’t reuse a pregnancy test. Let’s take a closer look at why.

Exactly how HPTs detect hCG is a trade secret of sorts, but we do know that they all work similarly — through a chemical reaction between your urine and the hCG antibodies in the strip. Once this reaction has taken place, it can’t occur again.

This goes for the digital ones, too. Although you don’t see a color-change strip or lines filling with blue or pink dye, it’s there, built into the test. The digital component of the test simply “reads” the strip for you and reports the results on a digital display screen. So you can’t reuse digital tests, either.

Generally speaking, you should read the results of a pregnancy test about 5 minutes after you POAS (pee on a stick in TTC-lingo) or dip it in urine and then discard it — and no pulling it out of the wastebasket an hour later, either! (Evaporation may have created a second line by that point, potentially making for a confusing and heartbreaking false positive.)

You may know from high school chemistry (or not — we don’t remember, either) that a chemical reaction between two agents happens once. Then, to accurately conduct that reaction again, you need to start fresh again with the same two agents.

So when your urine touches an HPT pee stick — either by you holding the stick mid-stream or dipping the stick into your collected urine — the reaction takes place. It can’t take place again. (Think of a kernel of corn popping — once it’s popped, you can’t pop it again. You need a new kernel.)

What if you open the test and it accidentally gets splashed with plain old water?

Well, remember that water is still made up of chemical elements — hydrogen and oxygen — that can react with the test strip. Presumably, water will give a negative result (we hope!), but you still can’t then add your urine to the strip as well.

If you reuse a strip that has gotten wet — either with water or urine and even if it’s dried — you may get a false positive.

That’s because as an HPT dries, an evaporation line can appear. Although this line is colorless, when you add more moisture to the stick, dye can settle in the evap line — forming what appears to be a positive.

Beyond that, a used test is considered a finished test. So any result you get from using it again should be seen as unreliable.

Always consult the instructions on the packaging. But this general procedure holds true for many of the most popular brands:

  1. Wash your hands. If planning to use the cup method, sterilize a cup with hot, soapy water.
  2. Unwrap an individual test and place on a clean, dry surface next to the toilet.
  3. Choose your method: For the cup method, start peeing, stop mid-stream and position the cup before restarting your stream and collecting enough for dipping (but not submerging) the stick in. Then dip the end of the test strip (not beyond the max line) into the cup of urine, holding it there for about 5 seconds. For the mid-stream method, start peeing, then position the test strip in your stream for about 5 seconds.
  4. Walk away (easier said than done) and let the chemical reaction take place.
  5. Come back to read the test 5 minutes later. (Let no more than 10 minutes pass. After 10 minutes, consider the test inaccurate.)

Again, check individual packaging, as some brands may differ.

It can be tempting to reuse a pregnancy test, especially if you’re sure that negative is incorrect, if you only got it a little wet, or if it’s dried since you’ve taken it and you’re out of tests.

But don’t give in to this temptation: Tests aren’t accurate after they get wet, either with your pee or with water.

If your test is negative and you still believe you’re pregnant, take heart. It can take a while for hCG to build up to detectable levels. Throw the used test away, try to get your mind off TTC, and test again with a new strip in 2 days’ time.