In the Lab: Why pregnancy tests need careful handling

Pregnancy tests are one of the most widely used rapid diagnostics in the world but the science behind them, and the common pitfalls in their use, are often less understood.

At SureScreen Diagnostics, we design and manufacture these tests. Here’s a closer look at how pregnancy tests work and why technique matters more than many people realise.

The hormone behind every pregnancy test

Pregnancy tests detect human chorionic gonadotropin (hCG), a hormone produced shortly after conception in the placenta. hCG promotes the release of progesterone, which signals the body to stop menstruating.  

Levels rise rapidly in early pregnancy, moving from just a few milli-international Units per millilitre (mIU/mL) to well over 200,000 mIU/mL as pregnancy progresses. The IU is the amount of protein needed to cause a specific level of activity, and we use it in this case like a mass concentration. We can think of this as ‘the amount of protein’.

Typical non-pregnant hCG levels:

• Women – less than 5 mIU/mL
• Men – less than 2 mIU/mL

But hCG isn’t exclusive to pregnancy. Certain cancers including testicular and placental cancers, and some medical conditions can elevate it too. That’s why accurate testing requires careful interpretation within clinical context.

How pregnancy tests “capture” hCG

Like many modern rapid diagnostics, pregnancy tests use a lateral flow sandwich assay. A urine sample travels along the strip where specially designed antibodies bind to hCG. This forms a “sandwich” between:

• Detection antibodies, labelled with red-coloured gold nanoparticles
• Capture antibodies, fixed at the test line

If hCG is present, the nanoparticles accumulate at the test line, producing the visible red result familiar to users. A second control line confirms that the reagents flowed correctly across the strip.

Cassettes vs dipsticks: why it matters

Most consumers assume all test formats behave the same, but dipsticks and cassettes work differently:

• Cassettes regulate the urine volume absorbed, minimising user error
• Dipsticks rely on correct immersion depth and time

And that’s where the most common problem arises.

Cassettes vs dipsticks: why it matters

Most consumers assume all test formats behave the same, but dipsticks and cassettes work differently:

• Cassettes regulate the urine volume absorbed, minimising user error
• Dipsticks rely on correct immersion depth and time

And that’s where the most common problem arises.

The number one cause of false positives: over-dipping

Over-dipping a dipstick can cause several issues inside the strip:

• Uncontrolled gold release

Excess urine can flood the conjugate pad, releasing too much labelled gold and increasing the likelihood non‑specific binding at the test line.

• Disrupted buffer chemistry

Pregnancy tests are designed to compensate for normal variations in urine pH and salt concentration.

When too much urine enters the strip, the chemistry can’t stabilise these changes, increasing the risk of a false positive result.

• Higher total hCG load

Even in non-pregnant individuals, hCG can be 5-10 mIU/mL first thing in the morning.

If a dipstick absorbs far more than the intended amount of urine, the test may receive enough total hCG to cross the positive threshold.

In other words: even “not pregnant” urine can appear positive if too much is absorbed.

Understanding results in real world settings

When interpreting any diagnostic test, it’s important to look beyond the laboratory numbers and understand how a test performs in everyday use.

The bottom line

Pregnancy tests are incredibly sensitive tools built on refined biochemistry, but their performance relies on correct technique, especially with dipstick formats.

At SureScreen Diagnostics, we combine scientific rigour with real world design insight to ensure our tests perform not only in the lab, but in the hands of everyday users.

For more information, contact our team: 

E: info@surescreen.com
T: 01332 365318