What is the recommended method for testing vitamin C (ascorbic acid) levels?

Recommended Method for Testing Vitamin C Levels

The preferred method for testing vitamin C status is through plasma vitamin C analysis, as it provides the most practical and reliable assessment while requiring less sample volume and fewer preanalytical steps than leukocyte testing. 1

Sample Collection and Processing Requirements

• Plasma samples (not serum) must be collected using lithium heparin tubes 1
• Immediate processing is essential due to vitamin C's high susceptibility to degradation:
  • Samples must be centrifuged immediately at 4°C 1
  • Plasma must be separated promptly after blood drawing 1
  • Samples require protection from light exposure throughout the entire workflow 1
  • Storage at ultra-low temperature (-70 to -80°C) is necessary 1

Analytical Methods

• High-Performance Liquid Chromatography (HPLC) is the gold standard method for vitamin C analysis 1, 2

  • Can be performed with ultraviolet (UV), fluorescence, or electrochemical detection 1
  • Requires acidification followed by immediate cold storage at ultra-low temperature 1
  • Can measure both L-ascorbic acid (AA) and its oxidation product dehydroascorbic acid (DHAA) 1

• Total vitamin C analysis typically involves:

  • Addition of a reducing agent (dithiothreitol or dithioerythritol) to convert DHAA into AA 1
  • DHAA can be determined by subtracting AA levels before reduction from total vitamin C 1

• Alternative point-of-care testing:

  • A newer point-of-care device measuring blood static oxidation-reduction potential (sORP) can estimate plasma vitamin C concentration 1
  • sORP is measured in non-acidified, non-reduced plasma within 20 minutes after centrifugation 1

Interpretation Challenges

• Vitamin C plasma levels decline rapidly with inflammation, making interpretation difficult 1
• Blood levels decrease when C-reactive protein (CRP) >10 mg/L 1
• Normal values are typically not detected if CRP >40 mg/L 1
• Measurement of plasma vitamin C is not recommended during critical illness or severe inflammation due to interpretation difficulties 1, 3

When to Test Vitamin C Levels

• Testing is recommended in patients with clinical suspicion of scurvy or chronic low intake 1, 3
• A clinical trial of vitamin C (approximately 1 g/day for at least one week) should not be delayed if clinical symptoms of deficiency are present 1

Common Pitfalls and Caveats

• Serum determination should be avoided due to poorer reliability 1
• Leukocyte vitamin C testing, while theoretically more indicative of tissue stores, has significant practical limitations:
  • Requires more sample volume 1
  • Involves more complex preanalytical steps 1
  • Shows variability between different cell types 1, 4
• Failure to follow strict sample collection and processing protocols can lead to falsely low results due to vitamin C's instability 1, 2
• Sex, age, smoking status, and medications can affect plasma vitamin C levels 4, 5