Vitamin B2 (Riboflavin)

What is Vitamin B2 (Riboflavin)?

A vitamin B2 (riboflavin) test measures how much of this B vitamin is circulating in your blood, which mostly reflects what you've eaten recently rather than your long-term stores.

Riboflavin is a water-soluble B vitamin your body uses to build two coenzymes, FAD and FMN, that drive dozens of energy-production reactions and help activate other B vitamins, including B6 and folate. You get it from milk, eggs, lean meat, green vegetables, and fortified grains, and because it's water-soluble, your body holds on to very little of it. Whatever you don't use gets cleared out in urine within a day or two, which is part of why a single blood draw is such an imperfect snapshot. That imperfection is worth naming directly, because it's easy to assume a blood test is automatically the most rigorous way to check a nutrient. For riboflavin, it isn't. Clinical laboratory science has long recognized a gap between a direct blood measurement and a functional test called EGRAC, the erythrocyte glutathione reductase activation coefficient. EGRAC doesn't measure riboflavin itself. It measures how much more active a riboflavin-dependent enzyme in your red blood cells becomes when a lab adds extra riboflavin cofactor to the sample. If the enzyme's activity jumps a lot, your cells were running short on riboflavin to begin with; if it barely moves, they already had enough. The NIH's Office of Dietary Supplements describes EGRAC as a stable and sensitive measure of riboflavin deficiency, with results read as adequate (1.2 or below), marginal (1.2 to 1.4), or deficient (above 1.4). A direct plasma or whole-blood riboflavin level, by contrast, is a simpler test to run but a cruder one to interpret, since it swings with what you ate and whether you fasted before the draw. Deficiency on its own is uncommon in people who eat a varied diet, since riboflavin shows up in so many everyday foods. It's more of a concern for people who eat very little animal-sourced food, athletes with high energy turnover and low intake, and people who are pregnant or breastfeeding, whose requirements rise. A rare inherited riboflavin transporter deficiency is a separate, distinct condition. None of this is something a single test result diagnoses on its own; it's context for a conversation with a doctor who knows your diet and health history.

Typical reference context

Ranges vary by lab and method, so always read your own report against the range printed on it, not a number from anywhere else.

What changed recently

The most current, best-evidenced development for riboflavin isn't about deficiency testing at all. It's about migraine. A dose-response systematic review and meta-analysis published in January 2026, covering 12 clinical trials and 749 participants, found that riboflavin supplementation (most commonly 400 mg a day, with doses studied from 50 to 400 mg, typically over 12 to 16 weeks) was associated with a meaningful drop in migraine frequency, about 1.39 fewer attacks a month compared with control groups, and a similar reduction in migraine duration. It did not meaningfully reduce attack severity. The reviewers called riboflavin an inexpensive, readily available, and safe supplement for improving frequency and duration, while flagging small trial sizes and inconsistent control designs, and calling for more research before it becomes a settled clinical recommendation. This is genuine, dated research on riboflavin itself, not a Longetic recommendation, and it doesn't change what a riboflavin blood test measures or how to read one.

Tests often checked alongside this

Riboflavin is rarely the only marker on a nutritional panel. Because FAD and FMN are needed to activate other B vitamins in the body, riboflavin is sometimes checked alongside a broader B-vitamin panel rather than on its own, particularly when a doctor is investigating unexplained fatigue or a suspected broader nutritional gap rather than looking at riboflavin in isolation.

What can affect a result

Two things specific to this marker matter more than for most other tests in this glossary. First, riboflavin degrades on exposure to light, so labs collect and ship samples in amber, light-protected tubes and reject specimens that weren't handled that way. It's not something you control as the patient, but it's a real reason a result can occasionally need to be redrawn. Second, direct plasma and whole-blood levels are sensitive to recent intake: Mayo Clinic Laboratories notes that testing a non-fasting specimen, or recent use of a riboflavin supplement, can result in elevated plasma vitamin B2 concentrations that don't reflect your usual status, which is why most labs ask for a 12-hour fast before this particular draw.

What to ask your doctor

• Was this a direct riboflavin level or the EGRAC functional test, and does that change how the result should be read?
• Did I need to fast, and was the sample handled with light protection?
• Is a low or borderline result worth rechecking, or is it explained by diet or timing?

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