Most companies selling finger-prick blood tests will tell you it is as good as a needle in your arm. It usually isn't. It is good enough for a lot of things, useless for some, and the difference matters more than anyone selling it wants to explain.
So here is the whole picture, including the parts that cost us sales.
What a finger-prick sample actually is
You prick the side of a fingertip with a single-use lancet and let drops of blood fall into a small tube. That tube goes in the post to the same UKAS-accredited laboratory, on the same analysers, run by the same staff as every venous sample we handle.
The analysis is identical. What differs is the sample, and everything below follows from that.
Why it isn't as good as a venous draw
Three reasons, and they compound.
Volume. A finger prick gives 40 to 100 microlitres. A venous tube gives five to ten thousand. There is no room for a second attempt at anything, and some markers simply need more blood than a fingertip will give.
The blood is stressed on the way out. Squeezing a finger ruptures red cells. That is haemolysis, and it wrecks certain results rather than just reducing them.
Blood sits on the skin before it reaches the tube. Blood starts to clot the moment it leaves you. On a venous draw it goes straight from vein to tube. On a finger prick it pools on the skin, drips, and mixes late.
The numbers, from our own laboratory. They put their venous sample failure rate at under 1%. For finger-prick panels it runs between 3% and 20% depending on the test, and the most common reason by a distance is simply not enough blood. Two tests are far worse than that, and we don't sell them by finger prick at all — see below.
We publish those figures because they are the ones that decide whether this is right for you. They are our laboratory's own, for the tests we actually sell, rather than a general figure lifted from somewhere else.
What we will and won't sell you
Our laboratory's accreditation lists exactly which markers it will certify on a capillary sample. We only sell you the ones on that list. For some we go further and refuse even where the accreditation allows it.
Available as a finger-prick test
Hormones — Male, Hormones — Female, Thyroid Function, Lipid Profile, Iron Profile, Kidney Function, Liver Profile, Kidney and Liver, Cardiovascular Risk and Tired All The Time.
Kidney and Liver is the only one of those with no venous version. If you want those markers from a venous sample, buy Kidney Function and Liver Profile separately.
Venous only
- Full Blood Count. Accredited for capillary, and we still won't sell it. Our laboratory's own failure rate for a capillary full blood count is 20 to 30% — it clots before it reaches the tube. One customer in three or four paying twice is not a product.
- HbA1c. Around 10% failure by finger prick, and it needs a second, different tube filled in the same sitting. Filling one finger-prick tube properly is enough to ask of anyone.
- Folate. Runs on the same tube as the full blood count, so it carries the same problem.
- Glucose, magnesium, calcium, cortisol, creatine kinase, sodium and a handful of others. Accredited for venous blood only, so a capillary result would not be certified.
- Any panel containing a full blood count — the N=1 Panel, TRT Monitor, TRT Monitor Complete and Full Body Baseline. And Metabolic Risk, which contains glucose.
One test loses a marker
Liver Profile is 8 markers venous and 7 by finger prick, because AST is accredited for venous blood only. Every other profile reports the same markers either way. The liver markers inside Kidney and Liver are the same 7, for the same reason.
We could run AST anyway and put a footnote on your report. We don't, for two reasons. A result that arrives outside the accredited scope isn't a result you can rely on. And AST in particular is one of the markers most distorted by haemolysis, so a stressed finger-prick sample tends to push it up — which produces a frightening number that means nothing. We would rather report 7 markers you can trust than 8 where one is noise.
The product page shows the range and says which marker is venous-only, so you know before you buy.
Things that will ruin your sample
Most failures are avoidable and come down to the same handful of causes.
- Cold hands. The single biggest cause of not getting enough blood. Warm them properly under running water for a couple of minutes first.
- Dehydration. Drink normally through the day before, and a good glass of water half an hour before you collect.
- Squeezing. Milking the finger is what causes haemolysis. Let it drip.
- Hand cream, makeup, fake tan. Anything on the skin can end up in the sample, and pigments containing iron oxide interfere with iron results directly. Wash and dry your hands thoroughly, and use nothing on them afterwards.
- Taking too long. If the blood is trickling, stop, warm the hand again and use a fresh lancet on a different finger.
- Posting late in the week. Collect Monday to Thursday and post the same day, before the last collection. A sample sitting in the postal network over a weekend can degrade past the point where the laboratory can run it.
If your sample can't be tested
We send you one free replacement kit for that order. Any further replacement on the same order is £20.
Where the fault is ours or the laboratory's — a kit that arrives damaged, a kit that never arrives, a sample lost or mishandled at the laboratory — we replace it free of charge, with no limit.
Here is why the first one isn't free forever. When your sample reaches the laboratory we are charged for the test whether or not it can be run, and the kit and the postage are already spent. A replacement is a second kit and a second analysis, charged to us again. The free one is us absorbing that rather than passing it on, and the £20 does not cover what a second one costs us either.
So which should you choose?
Choose a finger prick if you want the result without arranging anything, if you are testing often enough that appointments become the friction, or if the test you want is on the list above and you would rather not pay for a clinic.
Choose venous if you want a full blood count, if you want the widest choice of add-on markers, if you have tried a finger prick before and struggled, or if this is a result you are going to make a decision on and you want the most robust sample available.
Add-on markers are always run on a venous sample. A finger prick doesn't have the volume for them.
Neither is the wrong answer. They are different trade-offs, and now you know what they are.
Anything not covered here, email contact@n1health.co.uk and we will tell you straight.