One Sentence Before A Blood Draw, And What It Does

The sentence, and who it is for

If you take a supplement containing biotin and you are having blood taken, tell the person taking it. That is the entire instruction. “I take a biotin supplement” is enough on its own, and the amount helps if you happen to know it. There is nothing to fill in, nobody to ring beforehand and no cost attached.

The reason is that biotin at supplement amounts interferes with a whole family of laboratory tests. The FDA states it plainly: biotin can significantly interfere with certain lab tests and cause incorrect test results which may go undetected. The last four words carry the weight. A wrong result that announces itself as wrong is an inconvenience. A wrong result that looks like an ordinary result is a different category of problem, and it is the category this whole subject belongs to.

It is worth being equally clear about what the sentence is not. It is not a warning that you are in danger. It is not a reason to stop a prescribed medicine, and it is not a reason to put off a test. It is a piece of information that only you have, and saying it out loud changes what everybody downstream is able to do with a number.

Where the sentence goes after you say it

The value of the sentence is that it travels. The person holding the needle is rarely the person who will read the result, and the laboratory that runs the assay is a third party again. Three links, and the information has to start at the first one because nobody else can supply it.

How much of that happens, and in what order, varies between clinics and between laboratories, and this desk is not going to pretend it knows the procedure where you live. What does not vary is that the chain cannot start without the first link, and the first link is two seconds of your time.

There is a reason the person holding the needle is the right person to tell rather than the last one. They are the only one present at the single moment the sample exists and is still attached to a conversation. Once the tube is labelled and gone, the number that comes back is just a number, and whatever you know about your own cupboard has no way of catching up with it. Two seconds spent at the chair are worth more than an hour spent afterwards.

The regulator is working the same problem from the other end. FDA’s final guidance to industry, issued in October 2020, tells device makers how to test their assays for biotin interference and how to report what they find to laboratories and to clinicians. That is a live, formal position rather than a lapsed notice, and it means the system is built to carry this information. It simply needs somebody to put it in at the top.

Why troponin is the test that gets named

Immunoassays come in two broad shapes, and biotin pushes the two in opposite directions. A review of the mechanism sets it out in a line: competitive immunoassays read falsely high, and sandwich immunoassays read falsely low. Which direction an error runs in is not a detail. It decides whether a wrong number raises an alarm or quietly cancels one.

Troponin is a sandwich assay, and troponin is the blood test used to help diagnose a heart attack. So the error there runs in the direction that hides a heart attack rather than inventing one, which is precisely why it is the test named first in every official document on the subject. A falsely high result gets investigated. A falsely low result looks like good news.

The National Institutes of Health records the worst documented outcome in its own words: a patient with a high intake of supplemental biotin died following a troponin test that gave a falsely low result, because the test was subject to biotin interference. One case, recorded by a federal body, and it is the reason this subject is on every page of this website rather than tucked into a footnote.

Nor is this a historical problem that has since been tidied away. FDA’s live page on the subject carries a table of cleared troponin devices that had not addressed the risk, and counted off that table row by row it lists seventeen devices from six named manufacturers. The page also records that the agency has continued to receive adverse event reports of falsely low troponin results.

Troponin is the headline but it is not the whole list. Thyroid function tests are the other group named repeatedly in the source material, and they are worth knowing about because thyroid testing is ordinary, scheduled and often repeated, which makes it the test most people reading this will actually have. The point is not to memorise a list of assays. It is that the same sentence covers every one of them at once, which is what makes it worth saying rather than researching.

The size of the gap, printed plainly

Here is where this website stops short of the sentence a seller might prefer. Nobody has published an interference study at the amount printed on this label, and that cuts against a scare as much as it cuts against a reassurance.

The study that measured the effect in healthy adults gave six volunteers 10 mg of biotin a day for seven days and then ran thirty-seven immunoassays on their samples. Interference showed up in nine of the twenty-three assays that use biotin, and in none of the fourteen controls that do not. Five of eight competitive assays read falsely high; four of fifteen sandwich assays read falsely low. That is the design and the result, and it is a clean piece of work.

The wider record runs from the same floor. Case reports of interference sit in a range of 10 to 300 mg a day, and even a single 10 mg dose has interfered with thyroid function tests administered within twenty-four hours. The Gelagen serving carries 5,000 mcg, the same as 5 mg. That is half the lowest amount at which interference has actually been demonstrated in healthy volunteers, and at the same time it is 167 times the 30 mcg an adult is judged to need.

Both halves are true, and neither of them is a conclusion. What follows from them is exactly the sentence at the top of this article, and the sentence costs the same whether the gap turns out to matter or not. That is the whole argument for saying it: it is the only advice here that is free of the question it cannot answer.

When to say it, and what else to mention

Five things make the sentence more useful, and none of them takes longer than the sentence itself.

• Say it when a test is booked, if you know in advance, and say it again when the sample is actually taken. The two are often different people.
• Name the supplement and the amount if the jar is to hand. “Five thousand micrograms of biotin, two gummies a day” is far more useful than “a vitamin”.
• Count everything you take, not just this one. Hair and nail products, multivitamins and B-complex formulas all carry biotin, and the panels add up.
• Hold on to the fact that this is a measurement question rather than a safety one. It concerns what a test reads, and it resolves when the supplement stops.
• Never stop a prescribed medicine on the strength of anything printed on a supplement label, including this one.

There is one more thing worth saying, because it is the part people get wrong in the other direction. None of this makes a large biotin figure dangerous. No Tolerable Upper Intake Level has been set for biotin, because there is no evidence in humans that it is toxic at high intakes. The problem biotin causes is with instruments, not with tissue, and instruments can be told.

Which leaves a short, unglamorous conclusion. A jar of gummies cannot promise you anything about your hair. It can give you one sentence that is worth carrying into a clinic, and that sentence works the same whether you bought this jar, a different jar, or none at all.

Sources for this article