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Testing of nonfasting specimens or the use of vitamin supplementation can result in elevated plasma vitamin concentrations. Reference values were established in patients who were fasting.
After consuming vitamin C, plasma values rapidly rise within 1 to 2 hours and reach peak concentration within 3 to 6 hours after ingestion.
Values below 0.2 mg/dL indicate significant deficiency.
Values greater than or equal to 0.2 mg/dL and less than 0.4 mg/dL are consistent with a moderate risk of deficiency due to inadequate tissue stores.
Values of 0.4 to 2.0 mg/dL indicate adequate supply.
The actual level at which vitamin C is excessive has not been defined. Values above 3.0 mg/dL are suggestive of excess intake. Whether vitamin C in excess is indeed toxic continues to be uncertain. However, limited observations suggest that this condition may induce uricosuria and, in individuals with glucose-6-phosphate dehydrogenase deficiency, may induce increased red blood cell fragility.