This test is performed by placing a drop of 2,6-dichloro-indophenol on the tongue and noting the time required for the color to change from blue to pink. Proponents claim that this indicates whether there is enough vitamin C in the body. However, three published studies have demonstrated that the test is not valid. for this purpose:
- One study found no correlation between lingual test results and serum vitamin C concentration in 50 male dental students .
- Another study examined whether the test reflected the amount of vitamin C in the plasma and white blood cells during periods of vitamin C depletion and supplementation. During the experiment, 11 healthy young men consumed a diet that provided less than 5 mg of vitamin C per day. (The Recommended Dietary Allowance is 60 mg per day.) In addition, they were given 60 mg per day for two weeks, 0 mg (a placebo) per day for four weeks, 600 mg per day for three weeks, and 0 mg per day for four weeks. The vitamin C levels in the plasma and white cells responded rapidly to changes in vitamin C intake. However, the lingual test results were unrelated to these changes. The researchers concluded that lingual vitamin C test values are not related to changes in vitamin C intake and do not reflect the vitamin C levels in plasma or white cells .
- The third study tested 17 volunteers with the Vitamin C Self Test Kit, after which
a sample of their blood was analyzed using high-pressure liquid chromatography. The test subjects also kept a food diary for the day preceding the tests. No significant correlation was found between the lingual test result and the serum concentration or dietary intake of vitamin C 
- Randolph P et al. Evaluation of the lingual ascorbic acid test. Journal of Oral Medicine 29:8-12, 1974.
- Leggott PJ et al. Response of lingual ascorbic acid test and salivary ascorbate levels to changes in ascorbic acid intake. Journal of Dental Research 65:131-134, 1986.
- Stults VJ et al. Evaluation of a lingual test for vitamin V status. Journal of Oral Medicine 42:229-232, 1987.
This article was revised on April 9, 2002.