Other Vitamin Tests: A, C, E, K, B1 and B6 — When They Are Used and Their Limits
When clinicians use blood tests for vitamins A, C, E, K, thiamin (B1) and B6, what each measures, and why inflammation, fasting and sample handling limit the results.
Content last updated . This page has not been medically reviewed.
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These tests are tools a clinician orders when there is a reason, such as a malabsorption condition, a restricted diet, an unexplained symptom or a high-risk situation. They are not routine health checks, and they are not a substitute for a medical assessment. Some, such as thiamin testing in a person who may have Wernicke encephalopathy, are time-critical: a European neurology guideline says total thiamine in blood should be measured immediately before thiamine is given, and that the condition should be suspected in any situation that could lead to thiamine deficiency. [1] The same guideline lists eye signs, cerebellar (balance and coordination) problems and an altered mental state or memory impairment among the signs. [1] If you or someone you know has these, especially with heavy alcohol use or a very poor diet, seek urgent medical care (in the US, call 911). This page is information only.Test by test
| Vitamin | Test | What sources say it is used for | Limits |
|---|---|---|---|
| A | Serum retinol | MedlinePlus: checks for too much or too little vitamin A; the US encyclopedia's example normal range is 30 to 120 mcg/dL (1.1 to 4.2 micromol/L); fasting for up to 24 hours may be requested. [2] | Blood retinol is kept in a narrow range by the body. [3] It falls during infection and inflammation, so low results can be misleading. [3] In population surveys of children, deficiency was overestimated when inflammation was not accounted for. [4] |
| C | Plasma or serum ascorbic acid | See the note below on what could not be verified. | Vitamin C degrades rapidly, so samples need immediate centrifugation and acidification. [5] Plasma vitamin C also falls with inflammation. [6] |
| E | Serum tocopherol | MedlinePlus: for people who may not be absorbing vitamin E, such as with a malabsorption disorder, and to check for excess. [7] A fast of 12 to 14 hours is probably needed. [7] | Normal levels vary with age. [7] Deficiency is very uncommon in healthy people. [7] |
| K | Serum phylloquinone (K1); PIVKA-II | A 2020 review: K1 in serum is the most common marker; PIVKA-II shows how much vitamin K the tissues are using and is used to support diagnosis of newborn vitamin K deficiency bleeding. [8] | K1 is affected by recent diet. [8] Prothrombin time (a clotting test) has been used wrongly as an indicator of vitamin K status. [8] |
| B1 (thiamin) | Whole-blood thiamine diphosphate; transketolase activity | A 2026 review: whole-blood thiamine diphosphate and the transketolase activity assay are the most reliable markers. [9] | Plasma thiamine reflects recent intake and is not a useful marker of status. [9] In infants, none of the markers reliably identified which individual children would respond to thiamine. [10] |
| B6 | Plasma pyridoxal 5'-phosphate (PLP) | The most commonly used direct marker. [11] | Best assessed with a combination of markers because inflammation and other factors confound it. [11] |
For vitamin D, B12 and folate see vitamin D deficiency and testing, B12 tests and folate blood tests.
Why results mislead
A 2023 review says plasma and serum concentrations face pitfalls mainly because inflammation diverts micronutrients out of the bloodstream, and that significant decreases occur for thiamin, folic acid, cobalamin and vitamins A, C and D. It says a marker of inflammation, C-reactive protein, must always be measured at the same time. [6] MedlinePlus adds that for B vitamins several things can affect results, including inflammation and certain medicines, and that medical experts are not sure what a normal level is for some of them. [12] Different labs have different ranges. [12]Clinical use vs consumer panels
A clinician orders a test with your history, diet, medicines and symptoms in mind. Consumer panels sold without that context are discussed in at-home vitamin test kits and nutrient panels.Frequently asked questions
Who needs a vitamin A blood test?
MedlinePlus says the test checks for too much or too little vitamin A in the blood. [2] A clinician decides whether it is relevant, for example with fat malabsorption. [2]
Why can a low vitamin A result be misleading?
Blood retinol falls during infection and inflammation because the carrier protein is a negative acute-phase reactant. [3]
Is a clotting test the same as a vitamin K test?
No. A 2020 review says clotting assays such as prothrombin time have been used erroneously as indicators of vitamin K status. [8]
Which sample is most useful for thiamin?
Whole-blood thiamine diphosphate or the transketolase activity assay, according to a 2026 review. Plasma thiamine is not useful for status. [9]
Do I need to fast for vitamin tests?
It depends on the test. MedlinePlus lists fasting for vitamin A (up to 24 hours) and vitamin E (12 to 14 hours). [2] [7] Follow your lab's instructions.
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
How to read the evidence labels
Labels used on this page: established nutritional role / deficiency treatment, mixed, insufficient evidence and Proper evidence not found.
Proper evidence not found
- Vitamin C test thresholds for everyday use: Proper evidence not found. We looked for a MedlinePlus or NHS page on a vitamin C blood test and for a guideline in PubMed and did not find one to cite, so no reference range is given. The NHS page on scurvy lists risk factors rather than a test. [13]
- Whether a routine test benefits adults without symptoms or risk factors: Proper evidence not found in the sources read for this page.
Safety and who should talk to a clinician
- Supplements and test results: certain supplements, such as biotin, can interfere with lab tests. [14] Tell the lab and the ordering clinician about everything you take.
- Do not act on a single number. Low results should lead to finding the cause. See malabsorption, celiac disease, IBD and vitamin deficiency.
- Excess is also tested. MedlinePlus says a higher than normal vitamin A value means excess (toxic levels) [2], and that high vitamin E levels usually come from taking too many supplements. [7] Vitamin A has pregnancy cautions; see vitamin A excess.
- Blood thinners: vitamin K and E are covered in vitamins and blood thinners. Ask the anticoagulation clinic before changing intake.
- Before surgery, in pregnancy, or with kidney or liver disease: discuss supplements and tests with the care team; see medication interactions and deficiency testing and its limits.
References
- Galvin R, Bråthen G, Ivashynka A, et al.; EFNS. EFNS guidelines for diagnosis, therapy and prevention of Wernicke encephalopathy. Eur J Neurol. 2010 (abstract read). Read 2026-10-23.
- MedlinePlus Medical Encyclopedia (US National Library of Medicine). Vitamin A blood test. Review date 10/1/2025. Read 2026-10-23.
- Biomarkers of Nutrition for Development (BOND)-Vitamin A Review. PubMed record. J Nutr. 2016 (abstract read). Read 2026-10-23.
- Approaches to Assess Vitamin A Status in Settings of Inflammation: Biomarkers Reflecting Inflammation and Nutritional Determinants of Anemia (BRINDA) Project. PubMed record. Nutrients. 2018 (abstract read). Read 2026-10-23.
- Stability of whole blood and plasma ascorbic acid. PubMed record. Eur J Clin Nutr. 2007 (abstract read). Read 2026-10-23.
- Pitfalls in the interpretation of blood tests used to assess and monitor micronutrient nutrition status. PubMed record. Nutr Clin Pract. 2023 (abstract read). Read 2026-10-23.
- MedlinePlus Medical Tests (US National Library of Medicine). Vitamin E (Tocopherol) Test. Read 2026-10-23.
- Laboratory assessment of vitamin K status. PubMed record. J Clin Pathol. 2020 (abstract read). Read 2026-10-23.
- Assessment of thiamine status in humans. PubMed record. Methods Enzymol. 2026 (abstract read). Read 2026-10-23.
- Assessment of Erythrocyte Transketolase, Whole Blood Thiamine Diphosphate, and Human Milk Thiamine Concentrations to Identify Infants and Young Children Responding Favorably to Therapeutic Thiamine Administration (Lao Thiamine Study). PubMed record. Curr Dev Nutr. 2024 (abstract read). Read 2026-10-23.
- Ueland PM, Ulvik A, Rios-Avila L, et al. Direct and Functional Biomarkers of Vitamin B6 Status. Annu Rev Nutr. 2015 (abstract read). Read 2026-10-23.
- MedlinePlus Medical Tests (US National Library of Medicine). Vitamin B Test. Read 2026-10-23.
- NHS (UK). Scurvy. Page last reviewed 27 September 2023. Read 2026-10-23.
- US Food and Drug Administration. Biotin Interference with Troponin Lab Tests: Assays Subject to Biotin Interference. Content current as of 06/21/2022. Read 2026-10-23.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.