Which laboratory test best assesses pyridoxine (vitamin B6) status and what is the normal reference range?

Laboratory Assessment of Vitamin B6 Status

Plasma pyridoxal 5'-phosphate (PLP) is the gold standard biomarker for assessing vitamin B6 status, with a normal reference range of 20–200 nmol/L (5–50 mg/L), and adequate status defined as levels above 30 nmol/L. 1

Preferred Testing Method

• Plasma PLP measurement is the recognized status biomarker because it correlates directly with pyridoxine intake and body stores 1
• PLP levels respond to dietary intake and reflect liver stores, reaching plateau within 6–10 days after changes in consumption 1
• Modern analytical methods use HPLC with pre-column derivatization and fluorescence detection, or HPLC coupled to tandem mass spectrometry for high-throughput capabilities 1

Sample Collection and Handling Requirements

• Critical pre-analytical considerations: PLP is relatively stable at 4°C or -40°C but degrades rapidly at room temperature and when exposed to light 1
• Rapid plasma separation after blood collection and immediate frozen storage are essential for reliable quantitation 1
• PLP can be measured in plasma (EDTA), serum, or erythrocytes 1

Normal Reference Range

• Standard range: 20–200 nmol/L (5–50 mg/L) 1
• Adequate status threshold: >30 nmol/L 2
• Values below 20 nmol/L indicate deficiency 1

When to Use Alternative Testing

Red blood cell PLP measurement should be used instead of plasma PLP in specific clinical scenarios:

• Patients with inflammatory conditions, where plasma PLP falls despite adequate stores 1
• Low albumin states (e.g., critical illness, liver disease), since PLP binds tightly to serum albumin 1
• Elevated alkaline phosphatase activity, which influences plasma PLP concentration 1

Red cell PLP measurements are more reliable than plasma measurements for differentiating true vitamin B6 deficiency from apparent deficiency in these conditions 1, 2

Alternative Biomarkers (Not Recommended for Routine Use)

While other biomarkers exist, they have significant limitations and are not retained for routine clinical assessment 1:

• Plasma total vitamin B6 derivatives
• Urinary 4-pyridoxic acid and total pyridoxine
• Urinary tryptophan metabolites after tryptophan load
• Erythrocyte aminotransferase activities (EAST/EALT) - confounded by impaired renal function and require fresh whole blood 1

Quality Assurance

• Standard reference materials with certified values for PLP were developed by the United States National Institute of Standards 1
• External quality assurance programs for PLP are available through providers such as Instand 1

Clinical Context for Testing

Testing should be reserved for patients with:

• Risk factors including chronic kidney disease (especially dialysis), chronic alcoholism, medications interfering with B6 metabolism (isoniazid, penicillamine, anticancer drugs, corticosteroids, anticonvulsants), critical illness, or pregnancy 2
• Clinical manifestations of deficiency: seborrheic dermatitis with cheilosis and glossitis, microcytic anemia, epileptiform convulsions, confusion, depression, or angular stomatitis 2

Avoid testing asymptomatic patients without risk factors, as low-normal values do not constitute deficiency and do not warrant supplementation. 2