Why a Blood Test Result Can Be Wrong

Quick answer

A wrong blood test result is usually caused by problems before the sample is even analysed: biotin supplements interfering with thyroid and hormone assays, rough blood draws falsely raising potassium, or rare antibodies tricking the test. The core takeaway: unexpected results should be repeated before acting on them, with the supplement list, draw technique and assay interference all reviewed.

Cause of a wrong result	What happens
Biotin (vitamin B7) supplements	Interferes with the immunoassay technology used for thyroid and hormone tests.
Rough draw, tight tourniquet, fist squeezing	Ruptures red cells, falsely raising potassium (pseudohyperkalaemia).
Rare interfering antibodies	Can trick the assay and fake a positive result entirely.
What to do about a surprising result	Repeat the test, and review the supplement list and draw technique first.

Key points

Most errors happen before analysis: the preanalytical phase dominates lab error.
Biotin supplements can distort thyroid and hormone immunoassays; pause them before testing if advised.
Rough draws and fist squeezing falsely raise potassium.
Unexpected results should be repeated, with supplements and draw technique reviewed.

Common reasons a blood test result is wrong

Most wrong results are not machine errors — reviews consistently find the majority happen before the sample ever reaches the analyser. This table compiles the causes covered in this guide.

Cause	What happens	Tests most affected
Preanalytical errors (collection, handling, transport)	The largest share of laboratory mistakes — wrong tube, delayed transport, mislabelling	Almost any test
Biotin (vitamin B7) supplements	Interferes with the immunoassay technology used for several hormone tests	Thyroid tests (free T4, TSH), some hormone assays
Hemolysis from a difficult draw	Ruptured red cells spill potassium and other contents into the sample	Potassium (falsely high); several other chemistry values
Heterophile antibodies	The patient’s own antibodies bind the assay’s antibodies, faking a signal	Immunoassays such as troponin
Not fasting when required	Food raises glucose and triglycerides before the draw	Fasting glucose, lipid profile
Hook effect (very high analyte levels)	Extremely high levels overwhelm the assay and read falsely low	Prolactin, hCG and similar sandwich immunoassays

See also our guide on the hook effect and false-negative results.

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.

Wrong-result myths vs facts

MYTH A wrong result means the laboratory machine malfunctioned.

Fact: The analyser is usually fine. Reviews of laboratory error consistently find most mistakes happen in the preanalytical phase — collection, handling and transport — or come from interferences such as supplements.

MYTH If one result is wrong, none of your results can be trusted.

Fact: Most abnormal results are real. Interference is specific to particular tests and situations — chasing “maybe it’s an error” for every unwelcome number delays real answers.

MYTH Supplements can’t affect blood tests.

Fact: Biotin, a common ingredient in hair, skin and nail supplements at doses well above dietary need, interferes with the immunoassay technology behind several thyroid and hormone tests — enough to swing results significantly.

MYTH Repeating the test immediately always settles a surprising result.

Fact: Repeating without removing the cause — the biotin supplement still being taken, the same rough collection technique — can simply repeat the error. List every supplement first, then repeat.

What matters most when a result looks wrong

Fit matters: a result that clashes with how you feel, or looks physiologically implausible, deserves a second look — most such cases have a specific, identifiable reason.
List every supplement before repeating a surprising thyroid or hormone result — biotin is the commonest culprit.
Confirm before acting: repeat the test properly rather than treating a single surprising number.
Raise it with the clinician who ordered it rather than dismissing a result that genuinely concerns you.

Practical notes

Tell the lab about all supplements — biotin in particular can throw off thyroid and hormone assays — as well as recent illness and medicines. Your doctor may advise pausing biotin before certain tests, but never stop a prescribed medicine on your own.

Bring previous reports so a sudden, inexplicable change stands out against your history. When a result clashes with how you feel, ask directly whether a collection or laboratory issue could explain it — re-draws exist for exactly this reason.

Read a surprising result as ‘needs confirmation’, not as a verdict — trends matter more than single numbers. A repeat sample, ideally at the same laboratory, is the standard next step when the result and the symptoms disagree.

Don’t act on one unexpected positive — especially with screening tests — until confirmatory testing is complete. Discuss the result with your doctor first; worry is natural, but decisions should wait for confirmation.

In India

In India, confirmatory tests usually cost more than the initial screening test, though prices vary by city and lab. Before paying for a repeat, ask the lab whether the first sample itself can be rechecked — sometimes it can, at lower cost.

Choose an NABL-accredited laboratory, where sample-handling standards reduce pre-analytical errors. Home sample collection is available in major cities, and the reference range printed on your own report is the one that counts.

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.

My screening test came back positive — does that definitely mean I have it?

No. Screening tests are designed to catch every suspicious case, so false positives are expected — confirmatory testing decides the matter. Discuss the result with your doctor before drawing conclusions.

My vitamin D result changed a lot between two labs — which one is right?

Possibly both — different laboratories use different methods and reference ranges, so the same blood can read differently. Compare each result only with the range on its own report, use one lab when tracking trends, and discuss the difference with your doctor.

The lab wants to re-draw my blood — did they mess up my sample?

Usually it means the sample was unsuitable — clotted, haemolysed or insufficient — and the lab is protecting you from a wrong result. A re-draw is a quality step, not an accusation.

Can being stressed before the blood draw change my results?

Stress, dehydration, recent exercise and even a tight fist during the draw can nudge some results. A calm, well-hydrated draw is best — and a big surprise on the report still deserves a repeat before anyone acts on it.

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

Important: If a result genuinely concerns you, raise it with the clinician who ordered it rather than dismissing or self-diagnosing it as a lab error.