In women taking transdermal estrogen supplementation, we regularly see extremely elevated estradiol levels in finger prick measurements.
In many cases, this is not a true elevation in the blood, but rather contamination from the skin.
How skin contamination can distort your results
More and more women are using transdermal hormone therapy, such as estrogen patches, gels or creams. At the same time, low-threshold blood tests via finger prick are growing in popularity. That seems like an ideal combination - but with estradiol, things regularly go wrong.
In practice, we often see unexpectedly high estradiol levels in women taking hormone patches. Not because their blood levels are really that high, but because of contamination from the skin. In this article we explain how this happens, why it occurs especially with finger prick tests and what you can do reliably.
What is the problem often
A finger prick involves taking capillary blood through the skin. That sounds simple, but when hormones are taken through the skin, that is the weak spot. Transdermal hormones not only migrate through the skin - they also stay on it. Estradiol can accumulate in the upper layers of skin, sebum and skin fat. When you then take a finger prick, the hormone can move from the skin or from the skin layers into the blood sample. The laboratory then measures not only what is in your blood, but also what was on your skin.
The result: a falsely elevated estradiol value.
Why does this happen especially with a finger prick?
With a venous blood draw (via the arm), blood is drawn directly from the bloodstream, without contact with the skin surface at the measurement site. With a finger prick, this is different:
| Finger prick | Venous blood sampling |
|---|---|
| Blood comes through the skin | Blood directly from the vein |
| Susceptible to skin contamination | Barely affected by skin |
| Small amount of blood | Larger, more stable sampling |
| Extra risk with hormone creams/plasters | Suitable for hormone monitoring |
Even washing hands is often not enough to remove all estradiol residues, because the hormone can settle in the skin.
What types of hormone therapy does this occur with?
We see this problem mostly with:
- Estrogen patches
- Estrogen gel
- Estrogen cream
- Compound (bio-identical) hormone preparations
- Sometimes also with testosterone creams (in women and men)
And it can occur:
- Even if the patch is not on the hand
- Even if you wear gloves when applying
- Even days after application
Why is this clinically relevant?
Falsely elevated estradiol levels can lead to:
- Unnecessary dose reduction
- Unwarranted concerns about 'too high levels'
- Incorrect conclusions about absorption or metabolism
- Client or physician anxiety
- Misinterpretation of symptoms (such as sensitive breasts, mood swings)
In other words: you make adjustments based on a measurement error.
What is reliable in estradiol use? The best choice is a venous blood draw!
For women taking transdermal hormones, this is the gold standard for estradiol measurement.
Advantages:
- No skin contamination
- More reliable for dosing and follow-up
- More comparable with reference values
If you are considering finger pricks anyway, here's what's important to know:
- An elevated value may be contamination.
- Especially extremely high or suddenly rising values are suspicious.
- Results should always be interpreted in context
Repeat with venous blood is prudent when in doubt
Summary
- Transdermal estrogens remain attached to and in the skin
- When finger pricked, this can contaminate the blood sample
- This frequently leads to falsely elevated estradiol levels
- Venous blood sampling is the only reliable method in this case
