Vitamin C is measured through a blood test that checks the level of ascorbic acid in your plasma or serum. A standard blood draw at a lab or doctor’s office is the most reliable method, and results are reported in micromoles per liter (µmol/L) or milligrams per deciliter (mg/dL). Levels at or above 50 µmol/L (about 0.9 mg/dL) are considered adequate, while anything below 11 µmol/L (0.2 mg/dL) signals severe deficiency.
The Standard Blood Test
The primary way to measure vitamin C is a plasma ascorbic acid test, sometimes called a serum ascorbic acid test. A healthcare provider draws blood from a vein, and the sample is sent to a lab where it’s analyzed using a technique called high-performance liquid chromatography (HPLC). This chromatographic method is considered the most accurate. Older, non-chromatographic lab techniques can produce biased results, so HPLC has become the standard.
Once collected, your blood sample needs careful handling. The vitamin degrades quickly at room temperature, so labs preserve it with a stabilizing acid and freeze it. When stored properly at very low temperatures, vitamin C in plasma remains stable for years, losing no more than about 1% annually. This matters because some labs batch-process samples rather than running them immediately.
How to Prepare for the Test
Your doctor will likely ask you to fast for 8 to 12 hours before the blood draw. During that time, drink only plain water. Don’t chew gum, smoke, or exercise, as these can subtly influence results. Most importantly, stop taking vitamin C supplements for at least 24 hours before the test, and let your provider know about any vitamins or supplements you take regularly. A large dose of vitamin C the night before can temporarily spike your plasma levels and mask a longer-term insufficiency.
Timing matters because of how your body handles vitamin C. Your kidneys filter it continuously, and once your blood is saturated, your body simply excretes the excess. At intakes above roughly 500 mg per day, nearly all the surplus is flushed out in urine. Plasma levels max out around 70 to 80 µmol/L regardless of how much more you consume. So a single megadose before a test won’t push your results dramatically higher than saturation, but it can obscure a pattern of inadequate daily intake.
What the Numbers Mean
Lab results for vitamin C fall into well-defined ranges based on large population studies, including the National Health and Nutrition Examination Survey:
- Adequate: 50 to 69 µmol/L (0.9 to 1.2 mg/dL)
- Inadequate: 23 to 49 µmol/L (0.4 to 0.9 mg/dL)
- Hypovitaminosis C (low): 11 to 23 µmol/L (0.2 to 0.4 mg/dL)
- Deficiency: below 11 µmol/L (below 0.2 mg/dL)
Levels below 0.2 mg/dL are consistent with severe deficiency and raise the risk of scurvy, a condition marked by bleeding gums, easy bruising, joint pain, and poor wound healing. Levels above 0.6 mg/dL generally indicate sufficient dietary intake. If your result lands in the “inadequate” zone, you’re not in immediate danger, but your body’s stores are low enough that increasing your fruit and vegetable intake or adding a modest supplement is worthwhile.
Urine Test Strips
You can buy urine dipstick strips that detect ascorbic acid in urine, but they serve a different purpose than a blood test. These strips grade vitamin C concentration in urine on a rough scale: negative (below 10 mg/dL), 1+ (10 to 24 mg/dL), 2+ (25 to 49 mg/dL), and 3+ (50 mg/dL or higher). A positive reading tells you your body is excreting vitamin C, which generally means you’re consuming enough. A negative reading could mean low intake, but it could also simply mean you haven’t eaten vitamin C-rich foods recently.
These strips were actually designed to flag a different problem. High urinary vitamin C can interfere with other dipstick readings, causing false-negative results for blood in urine and for markers of urinary tract infections. In one large multicenter study, about 10.6% of urine samples with detectable vitamin C gave false-negative results for blood, and 8.2% gave false negatives for infection markers. So while these strips can give you a ballpark sense of whether you’re excreting vitamin C, they aren’t a substitute for a plasma test if you want to know your actual status.
Who Should Get Tested
Most people eating a varied diet with fruits and vegetables don’t need formal testing. A daily intake of 200 to 400 mg of vitamin C is enough to fully saturate blood levels in healthy adults, and that’s achievable with a couple of servings of citrus, bell peppers, strawberries, or broccoli. The recommended dietary allowance for non-smoking adults is lower than that (75 mg for women, 90 mg for men), but those are minimum targets to prevent deficiency rather than goals for optimal levels.
Smokers are a notable exception. Smoking accelerates vitamin C breakdown in the body, and the recommended intake for smokers is about 35 mg per day higher than for non-smokers. Despite this, many smokers fall short. If you smoke, testing can reveal whether your levels have dropped into the inadequate or deficient range.
Testing also makes sense if you have digestive conditions that impair nutrient absorption (like Crohn’s disease or celiac disease), if you follow a very restricted diet, or if you’re showing possible signs of deficiency: unexplained fatigue, slow-healing wounds, swollen or bleeding gums, or frequent bruising. Older adults, people with alcohol use disorder, and those on very limited diets are at higher risk for low levels.
Getting the Test
You can request a vitamin C blood test through your primary care provider. Some direct-to-consumer lab services also offer it without a doctor’s order. The test itself is a simple blood draw and typically costs between $30 and $80 without insurance, depending on the lab. Results usually come back within a few business days. If your levels are low, the fix is straightforward: increasing dietary vitamin C or taking a daily supplement in the range of 100 to 250 mg is enough to bring most people into the adequate range within a few weeks.