Commercial Testing for Pyridoxal 5'-Phosphate (PLP)
Yes, commercial laboratory testing for plasma pyridoxal 5'-phosphate (PLP) is widely available and represents the gold standard biomarker for assessing vitamin B6 status. 1
Primary Testing Method
Plasma PLP measurement is the recommended first-line test for determining vitamin B6 status, as it correlates directly with vitamin B6 intake and reflects hepatic stores, typically plateauing within 6-10 days of supplementation 1
Modern commercial laboratories utilize high-performance liquid chromatography (HPLC) with fluorescence detection after pre-column derivatization, or more advanced HPLC coupled to tandem mass spectrometry (HPLC-MS/MS) for high-throughput, accurate measurements 1
Standard reference materials with certified PLP values have been developed by the United States National Institute of Standards, and quality assurance programs are available through providers like Instand, ensuring commercial test reliability 1
Normal Reference Range
Normal plasma PLP concentrations range from 5-50 mg/L (20-200 nmol/L) 1
Values below 30 nmol/L indicate borderline vitamin B6 deficiency and are associated with increased disease risk 2
Critical Testing Considerations
When Standard Plasma Testing May Be Unreliable
In seriously ill patients or those with inflammation, red blood cell (RBC) PLP measurement should be ordered instead of plasma PLP 1, 3
Inflammation from various conditions causes plasma PLP levels to fall artificially, even without true vitamin B6 deficiency 1, 4
Plasma albumin concentration and alkaline phosphatase activity directly influence plasma PLP measurements, making them unreliable in conditions with low albumin (such as inflammatory states) 1, 3
RBC PLP measurements are more reliable for differentiating true vitamin B6 deficiency from apparent deficiency in these clinical contexts, as they reflect long-term status tied to erythrocyte lifespan and are minimally affected by inflammation 1
Sample Handling Requirements
PLP is relatively stable at 4°C or -40°C but degrades rapidly at room temperature and when exposed to light 1
Commercial testing requires rapid plasma separation after blood collection and frozen storage to ensure accurate results 1
Blood samples should be processed within one hour when possible, as demonstrated in hypophosphatasia studies 5
Alternative and Supplementary Tests
Erythrocyte PLP can be measured when plasma testing is contraindicated, though it requires fresh whole blood sampling 1
The PLP-to-pyridoxal (PL) ratio is available commercially and may provide additional diagnostic information, particularly for monitoring certain conditions like hypophosphatasia 5
Functional tests including erythrocyte aminotransferase activities (EAST, EALT) exist but are less practical for routine clinical use 1
Clinical Pitfalls to Avoid
Do not rely solely on plasma PLP in patients with: acute or chronic inflammation, low serum albumin, altered alkaline phosphatase activity, critical illness, or renal dysfunction 1, 3
Medications that can affect results include: isoniazid, penicillamine, anti-cancer agents, corticosteroids, and anticonvulsants—all of which inhibit vitamin B6 activity 1
Plasma PLP may be suitable for determining vitamin B6 status at the population level but cannot reliably predict tissue PLP concentrations in individual patients 6