Pregnancy test sensitivity: what 10 or 25 mIU/ml means

Some test boxes show a number like 10 or 25 mIU/ml. It tells you how sensitive the test is. Here’s what that means, and why it matters most if you test early.

What the number means

mIU/ml (milli-international units per millilitre) measures how much hCG is in your wee. You’ll also see it written as IU/L, which means the same thing. The lower the number on the box, the less hCG the test needs to show a positive.

For example, one brand’s early test says it detects 10 mIU/ml, and the urine pregnancy test used by one NHS hospital lab detects 25 IU/L.

How much hCG is there when your period is due?

It varies hugely. A 2004 US study measured hCG in urine around the time of a missed period. The average was 49 mIU/ml, and in 9 out of 10 pregnancies it was between about 12 and 240 mIU/ml. About 1 in 20 had less than 12.

And some pregnancies have no hCG at all yet. One study found that on the day women expected their period, about 1 in 10 pregnancies hadn’t implanted, so no test could have found them.

That’s why two people at the same point in their cycle can get different results, and why a test that worked for a friend at 10 days past ovulation might not work for you.

Why levels vary

When you implanted. Implantation happens anywhere from about 6 to 12 days after ovulation. Later implantation means less hCG by the time your period is due.
When you ovulated. It’s hard to pinpoint. If you ovulated later than you thought, you’re earlier in the pregnancy than you think.
How diluted your wee is. Lots of drinks before testing can water it down. Your first wee of the morning is the most concentrated.
Where the pregnancy is. Ectopic pregnancies often make lower levels of hCG.

Does the number on the box tell the whole story?

Not always. In the same 2004 study of 18 brands, only 44% picked up 100 mIU/ml at the reading time in their instructions, and only one picked up 12.5 mIU/ml. Tests have changed since then, but they still vary in how well they detect low levels of hCG.

Whatever the box says, a negative before or on the day your period is due doesn’t rule out pregnancy. Always read the result within the time the instructions give, as a line that appears later may not mean anything.

Choosing a test

If you want to test before your period is due, a more sensitive test gives you a better chance of an early positive. But no test can pick up hCG that isn’t there yet.

Some early tests can be used from 5 days before your period is due (the packet may say ‘6 days before your missed period’), but they won’t pick up every pregnancy that early. Testing from the day your period is due is more reliable, though a negative can still be wrong if there isn’t enough hCG yet. If your period doesn’t come, test again a few days later.

Try not to compare lines

Line darkness varies between brands, batches and people. Even a faint line usually means a positive result. If you’re unsure, wait a couple of days and test again, as hCG usually rises quickly in early pregnancy. See what does a faint line mean?

If you’re worried about your levels

Home tests can’t tell you how your hCG is changing. If you have bleeding or tummy pain, get urgent advice (see below). An early pregnancy unit can measure your hCG in two blood tests, 48 hours apart, if they need to. Read about what those results mean in hCG: what it is and how it works.

After fertility treatment

If you had a ‘trigger’ injection containing hCG, it can stay in your body for up to about 10 days and give a positive that isn’t a pregnancy. Test on the date your clinic gives you, as testing earlier may give a false result.

Questions people ask

Is a 10 mIU/ml test better than a 25 mIU/ml test?

It’s more sensitive, so it may pick up a pregnancy a little earlier. From the day your period is due, either will pick up most pregnancies, but a 25 mIU/ml test may miss a few more on that day. If it’s negative and your period doesn’t come, test again a few days later.

Can hCG be too high for a test to work?

Very rarely. At very high levels, hCG can stop a test’s antibodies working properly and give a negative. This is called the hook effect. If you have pregnancy symptoms and keep getting negatives, see your GP.

What hCG level means you’re pregnant?

At one NHS hospital lab, a blood level above 25 IU/L means pregnancy, and 10 to 25 is unclear, so the test is repeated after 24 hours. Other labs may use slightly different cut-offs. For home tests, check the box or leaflet for your brand’s sensitivity (1 mIU/ml is the same as 1 IU/L).

When to get help

Mild cramps that ease with rest are usually nothing to worry about. But if you’re pregnant, or think you could be, get urgent advice from your maternity unit, an early pregnancy unit or NHS 111 if you have:

any bleeding or spotting, even if it’s light
tummy pain that’s severe, on one side, or hasn’t eased after resting for 30 to 60 minutes
pain in the tip of your shoulder

Call 999 if you have bleeding along with severe tummy pain, pain in your shoulder, or feeling sick, faint or dizzy, or if the bleeding is heavy (soaking a pad soon after putting it on). Also call 999 if you have sudden, sharp and intense tummy pain and you feel very dizzy or faint, feel sick or look very pale. These can be signs of an ectopic pregnancy or a miscarriage that need treating straight away.