Why a Blood Test Result Can Be Wrong

Most Errors Happen Before the Blood Is Even Tested

It's tempting to picture a wrong result as a machine malfunctioning. In practice, reviews of laboratory error consistently find that the majority of mistakes happen in the preanalytical phase – everything that happens to the sample before it ever reaches the analyser, from how the blood was drawn to how the tube was labelled, stored and transported – rather than during the actual measurement.¹ That distinction matters practically: a discordant result is far more likely to trace back to something about the sample or the person than to the analyser being faulty.

Biotin Supplements Can Throw Off Thyroid and Hormone Tests

Biotin (vitamin B7), a common ingredient in hair, skin and nail supplements taken at doses well above dietary need, interferes with a specific immunoassay technology used for several thyroid and hormone tests. A study measuring this directly found significant swings in Free T4, Free T3 and total T3 within two hours of a single biotin dose, while TSH and total T4 – which use a different assay design – were unaffected in the same samples.² The telltale pattern is a Free T4/T3 that looks hyperthyroid while TSH stays normal, which doesn't add up physiologically and should prompt a question about supplements before it prompts a diagnosis. Most labs now advise stopping high-dose biotin for at least 2 days, and sometimes longer, before affected blood tests.

A Squeezed Fist or Rough Draw Can Fake a Potassium Problem

Red blood cells hold far more potassium inside them than sits in the surrounding plasma. A traumatic draw, a tourniquet left on too long, or squeezing and releasing a fist repeatedly during the draw can rupture some of those cells – a process called hemolysis – releasing potassium into the sample and pushing the reading up without any real change in your body's potassium. Laboratory researchers have found this effect predictable enough to build correction formulas for it, and a genuinely high potassium result caused by hemolysis is common enough that many labs flag and recheck it automatically before it's reported.³

Rare Antibodies Can Fake a Positive Result Entirely

A concrete example: false-positive troponin

Cardiac troponin is the test used to rule myocardial injury in or out, and a false positive here isn't a minor inconvenience – it can trigger unnecessary cardiac procedures. In a cohort of patients previously diagnosed with myocarditis whose troponin stayed persistently, atypically elevated without matching symptoms or ECG changes, roughly one in three tested positive for troponin assay interferents – heterophile antibodies or troponin macrocomplexes that the immunoassay mistakes for the real target. Identifying the interference changed management in nearly half of those cases.⁴

Heterophile antibodies are proteins some people's immune systems produce that happen to bind to the antibodies used inside an immunoassay, generating a signal the test reads as a real result. They're uncommon, but when a result is markedly abnormal and doesn't fit the clinical picture at all, this kind of interference is one of the specific things a laboratory can test for – often by re-measuring on a different assay platform or using a chemical step that strips out interfering antibodies but leaves the true analyte intact.

What this doesn't mean

None of this is a reason to distrust results generally, or to assume every abnormal test is an error. It's the opposite: most abnormal results are real, and chasing "maybe it's interference" for every unwelcome number delays real answers. This matters specifically when a result is markedly discordant with your symptoms, history and exam – that specific mismatch is the signal worth raising with your clinician.

Frequently Asked Questions

How common are false positive blood test results?

Genuine laboratory analytical errors are rare, but interference from something other than the analyte itself is not: reviews of laboratory error put the majority of mistakes in the preanalytical stage, before the sample is ever tested, rather than in the analysis itself.

Can biotin supplements really affect a blood test?

Yes, for tests that use biotin-based immunoassay technology, which includes several thyroid and hormone tests. Studies measuring this directly have found significant swings in results within hours of taking a biotin supplement. Labs generally advise stopping high-dose biotin for at least 2 days before affected tests.

Why would a squeezed fist during a blood draw change the result?

A difficult draw, a tight tourniquet left on too long, or a squeezed fist can rupture red blood cells in the sample (hemolysis), releasing their contents into the surrounding fluid. Potassium sits at a much higher concentration inside red blood cells than in plasma, so even mild hemolysis can push a potassium reading up without reflecting your body's true level.

What should I do if a result doesn't match how I feel?

Say so. Tell your clinician the result doesn't match your symptoms or history, ask whether a repeat test or a different assay platform would help, and mention any supplements (biotin especially) or recent illness. Clinical-laboratory discordance is a recognised reason to investigate interference, not something to just accept.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

De la Salle B. Pre- and postanalytical errors in haematology. Int J Lab Hematol. 2019;41 Suppl 1:170–176. PMID 31069968 · doi:10.1111/ijlh.13007
McBride M, Dasgupta A. Significant Interference of Biotin in Thyroid Function Tests Using Beckman Analyzer: How to Identify such Interferences? Ann Clin Lab Sci. 2023;53(1):130–133. PMID 36889777
Lee TS, Kim J, Uh Y, Park YC, Yoo GS, Yoon KJ. Correction Equation for Estimation of Actual Potassium Concentration in Hemolyzed Specimen. Clin Lab. 2017;63(2):271–275. PMID 28182346 · doi:10.7754/Clin.Lab.2016.160733
Menghi C, Giordani AS, Baritussio A, et al. Cardiac troponin interferents in diagnosis and clinical management of myocarditis: a single center experience. Int J Cardiol. 2025;446:134087. PMID 41412386 · doi:10.1016/j.ijcard.2025.134087

When a Blood Test Result Is Wrong: False Positives, Interference and Lab Error