Vitamin C (Ascorbic Acid) Test
The Vitamin C test measures plasma ascorbic acid concentration to assess deficiency, monitor supplementation, and evaluate nutritional status in clinical practice. It is indicated for suspected scurvy, unexplained fatigue, poor wound healing, immune compromise, malabsorptive conditions, and high oxidative stress states including smoking. Specimen handling requires immediate plasma separation and freezing due to the labile nature of ascorbic acid.
1
Biomarkers
Blood (plasma)
Sample
3–5 business days
Turnaround
No — however, practitioners should document recent vitamin C supplementation as it significantly affects plasma levels
Fasting
What is the Vitamin C test?
The Vitamin C Test measures plasma ascorbic acid concentration, providing a direct assessment of vitamin C status in the body. It is ordered when clinical presentation suggests deficiency — including unexplained fatigue, gingival bleeding, petechiae, poor wound healing, recurrent infections, or musculoskeletal pain — and to monitor repletion in patients undergoing supplementation protocols. Plasma measurement reflects recent intake and is the most practical and clinically validated index of vitamin C status in the ambulatory setting. Within the nutritional assessment landscape, plasma vitamin C is the standard first-line test for deficiency investigation. Leucocyte ascorbic acid measurement provides a more accurate reflection of tissue stores but is not routinely available through commercial laboratory networks; plasma remains the clinical standard. This test is particularly relevant in populations at elevated risk — smokers, individuals with limited dietary variety, those with malabsorptive conditions, critically ill patients, and those with high oxidative stress burden — and integrates naturally into broader micronutrient and functional health panels.
What does the Vitamin C test measure?
Ascorbic acid is a water-soluble antioxidant vitamin that humans cannot synthesise endogenously due to the absence of the enzyme L-gulonolactone oxidase. It is absorbed in the small intestine via sodium-dependent vitamin C transporters (SVCT1/SVCT2) and circulates primarily in plasma, with tissue concentrations substantially higher than plasma — particularly in the adrenal glands, leucocytes, and brain. Plasma half-life is approximately 10–20 days at physiological concentrations. The vitamin participates in collagen hydroxylation, catecholamine synthesis, iron absorption enhancement, and broad antioxidant defence.
Plasma ascorbic acid levels directly reflect recent dietary intake and absorption status, making them the preferred marker for assessing vitamin C deficiency and adequacy. Levels below 0.2 mg/dL are associated with clinical scurvy, while subclinical deficiency (0.2–0.4 mg/dL) correlates with impaired collagen synthesis, immune compromise, and increased oxidative stress burden. Practitioners use this test to guide supplementation, monitor repletion, and investigate unexplained fatigue, poor wound healing, or inflammatory states where antioxidant depletion is suspected.
Why is the Vitamin C test ordered?
- Suspected clinical or subclinical vitamin C deficiency — unexplained gingival bleeding, petechiae, perifollicular haemorrhage, or slow wound healing without clear aetiology
- Evaluation of fatigue and immune compromise in patients with limited fruit and vegetable intake or restrictive dietary patterns
- Monitoring vitamin C repletion in patients on supplementation protocols, particularly following confirmed deficiency
- Pre- and post-operative nutritional assessment where wound healing optimisation is clinically relevant
- Assessment of nutritional status in patients with malabsorptive conditions — Crohn's disease, coeliac disease, short bowel syndrome, or post-bariatric surgery
- Smokers and individuals with high oxidative stress burden — smoking significantly increases ascorbic acid turnover and depletion rate
- Integrative and functional medicine practitioners building comprehensive micronutrient profiles alongside zinc, magnesium, and B-vitamin panels
- Investigation of unexplained musculoskeletal pain or connective tissue fragility where collagen synthesis impairment is suspected
Sample collection and turnaround
Sample type
Blood (plasma)
Fasting required
No — however, practitioners should document recent vitamin C supplementation as it significantly affects plasma levels
Collection method
Phlebotomy via LabCorp draw site
Turnaround
3–5 business days
Collection notes
Collect into EDTA or sodium oxalate tube as specified by LabCorp protocol. Plasma must be separated from cells immediately and frozen within 30 minutes of collection. Specimen is highly labile — ascorbic acid oxidises rapidly at room temperature. Protect from light. Patient should avoid high-dose vitamin C supplementation for 24 hours prior to collection if baseline status (rather than peak plasma) is being assessed. # REVIEWER FLAG: Confirm exact tube type and processing protocol with LabCorp specimen handling guidelines before publication.
Specimen requirements
Minimum 1.0 mL plasma. Separate immediately and freeze at −20°C or colder. Transport on dry ice. Do not use serum — plasma is required for this assay.
What can affect Vitamin C results?
Cigarette smoke generates reactive oxygen species that accelerate ascorbic acid oxidation, increasing turnover and depletion. Smokers have plasma vitamin C levels approximately 25–40% lower than non-smokers at equivalent dietary intake.
Plasma ascorbic acid rises rapidly following oral supplementation and reflects recent intake rather than tissue stores. Practitioners should document supplementation status and timing relative to specimen collection.
Ascorbic acid is highly labile and oxidises rapidly in plasma at room temperature. Delay between collection and processing causes artifactually low results. Specimen must be processed promptly per protocol.
Systemic inflammation accelerates ascorbic acid consumption as part of the antioxidant response. Plasma levels may underestimate true intake-based status during acute inflammatory episodes.
Chronic aspirin and NSAID use can reduce plasma ascorbic acid through increased renal excretion and increased oxidative consumption, independent of dietary intake. # REVIEWER FLAG: Drug-interaction claim based on probabilistic clinical knowledge — reviewer to verify against pharmacological literature.
Hyperglycaemia competitively inhibits cellular uptake of ascorbic acid via GLUT transporters, reducing intracellular concentrations. Plasma levels may appear relatively preserved while tissue stores are depleted.
What do Vitamin C results mean?
Elevated
Plasma ascorbic acid above 2.0 mg/dL (114 µmol/L) typically reflects high-dose supplementation rather than dietary excess, as renal excretion limits accumulation. Toxicity from dietary sources is not clinically reported; supplemental excess at very high doses may be associated with oxalate nephrolithiasis risk in susceptible individuals. Practitioners should document supplementation history.
Normal
Plasma levels within the reference range (0.4–2.0 mg/dL) confirm adequate circulating ascorbic acid. In functional medicine contexts, practitioners may target the higher end of the range (0.8–1.6 mg/dL) as an optimal zone for antioxidant and immune function, though this is not standardised in mainstream guidelines.
Reduced
Levels below 0.4 mg/dL indicate deficiency; levels below 0.2 mg/dL are consistent with frank scurvy. Subclinical deficiency (0.2–0.4 mg/dL) warrants dietary assessment and supplementation guidance. Consider underlying malabsorption, increased metabolic demand (smoking, inflammation, critical illness), or inadequate intake as contributing factors.
Note: Plasma ascorbic acid reflects recent dietary intake rather than true body stores — a patient who has recently consumed high-vitamin-C foods or supplements may return a normal plasma result despite chronically poor status. Leucocyte ascorbic acid is a superior marker of tissue stores but is not routinely available. Specimen handling is critical; labile oxidation of ascorbic acid in improperly handled or delayed specimens will produce artifactually low results regardless of true status.
Frequently asked questions
Plasma is the required specimen type for ascorbic acid measurement. Serum is not appropriate because the clotting process and cell lysis during serum preparation lead to significant ascorbic acid degradation, producing unreliable results. All LabCorp vitamin C assays use plasma.
How to access the Vitamin C test
The Vitamin C reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: May 13, 2026
Last reviewed: May 13, 2026
- Carr AC, Maggini S. Vitamin C and Immune Function. Nutrients. (2017)
- Padayatty SJ, Levine M. Vitamin C: the known and the unknown and Goldilocks. Oral Dis. (2016)
- Lykkesfeldt J, Tveden-Nyborg P. The Pharmacokinetics of Vitamin C. Nutrients. (2019)
- Schleicher RL, et al. Serum vitamin C and the prevalence of vitamin C deficiency in the United States: 2003–2004 NHANES. Am J Clin Nutr. (2009)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.