Proper Use of RBC Folate Test
The RBC folate test should be used as a marker of long-term folate status in patients with macrocytic anemia or at risk of malnutrition, and should be measured using a method validated against the microbiological assay. 1
When to Use RBC Folate Testing
RBC folate testing is appropriate in specific clinical scenarios:
- In patients with macrocytic anemia
- In patients at risk of malnutrition
- In diseases known to increase folate requirements
- When evaluating for suspected folate deficiency
Advantages of RBC Folate Over Serum Folate
RBC folate offers distinct advantages as a biomarker:
- Reflects long-term folate status (previous 3 months) and tissue folate stores 1
- More stable indicator than serum folate, which only reflects recent dietary intake
- Better correlation with clinical manifestations of folate deficiency
Frequency of Testing
The ESPEN micronutrient guidelines provide clear recommendations on testing frequency:
- Initial assessment for patients with macrocytic anemia or malnutrition risk
- Repeat within 3 months after supplementation to verify normalization
- For diseases with increased folate needs: every 3 months until stabilization, then annually 1
Proper Specimen Collection and Handling
The accuracy of RBC folate testing depends on proper specimen handling:
- Whole blood samples should be stored at 4°C if testing will be delayed (stable up to 72 hours) 2
- At room temperature, samples remain stable for at least 24 hours 2
- Ascorbic acid solution must be used during hemolysis to protect folate from oxidation 1
- Preliminary hemolysis of whole blood by dilution with ascorbic acid solution is required 1
Recommended Testing Method
The gold standard method for measuring folate is the microbiological assay with Lactobacillus rhamnosus 1, 3. This method:
- Responds to all active monoglutamate forms of folate
- Provides the most accurate assessment of folate status
- Should be the reference against which other methods are validated
Other methods include:
- Protein binding assays (less reliable due to different affinities for folate forms)
- LC-MS methods (high precision but require specialized equipment)
Interpretation of Results
- Normal RBC folate level: ≥340 nmol/L 1
- Concurrent measurement of homocysteine improves interpretation
- Always measure vitamin B12 simultaneously when testing for folate deficiency 1
Important Caveats and Pitfalls
Vitamin B12 deficiency must be ruled out: Folate supplementation can mask B12 deficiency, improving blood counts while neurological manifestations worsen 1
Assay variability: RBC folate levels are assay-dependent, with significant variations between different testing methods 4
Pre-analytical factors: Antibiotics affecting L. rhamnosus can interfere with microbiological assay results 1
Limited clinical utility in some populations: In the post-fortification era, folate deficiency is rare in Western countries 5
Correlation with intake: RBC folate correlates with total folate intake but correlation coefficients range only from 0.27 to 0.46 4
When Not to Order RBC Folate Testing
RBC folate testing has limited utility in:
- Routine evaluation of dementia or altered mental status in the absence of anemia 5
- Patients without macrocytic anemia or risk factors for deficiency 5
- Areas with mandatory folate fortification of foods, where deficiency is rare 1, 5
By following these evidence-based guidelines for RBC folate testing, clinicians can appropriately assess folate status while avoiding unnecessary testing in populations where deficiency is unlikely.