What is a Realistic Upper Limit for Red Blood Cell Folate?

[Little Bluestem]

I recently had some testing done. The limits given for RBC folate is >280 ng/mL RBC. This makes my 592 within acceptable range. Is it really?

 

[AFCFS]

I recently had some testing done. The limits given for RBC folate is >280 ng/mL RBC. This makes my 592 within acceptable range. Is it really?

You think that would be a simple question.

Medscape Reference Folate (Folic Acid), gives this Reference Range: Adults: 140-628 ng/mL, or 317-1422 nmol/L.

Apparently there is some debate about the analysis, as noted in A Practical Approach to Red Blood Cell Folate Analysis. Although no range is give, in the two methods of compared analysis, they show:

The mean ± SD and median of RBC folate by the routine assay was 449 ± 216 ng/ml and 399 ng/ml. The mean ± SD and median of RBC folate by the new assay was 366 ± 188 ng/ml and 309 ng/ml.

Then there is this 24 page document from the CDC:
Laboratory Procedure Manual
Analyte: Folate
Matrix: Serum/Whole Blood
Method: Microbiological Assay
Method No: 4000.03

On page 17 and 18, after much ado (which is likely required to make sense of what is quoted), they state the following:

13. Reference Ranges (Normal Values)
Previously generated reference ranges for the U.S. population cannot be directly applied to this assay since they were produced with the BioRad assay which measures significantly lower than the microbiologic assay. Clinical text books provide some information on reference ranges with the microbiologic assay, however those data were produced before folic acid fortification was introduced in the U.S. Therefore we don’t have post-fortification reference ranges for the U.S. population with the microbiologic assay. NHANES 2007-2008 will be used to generate these reference ranges.

14. Critical Call Results (“Panic Values”)
Any samples with serum folate levels <7 nmol/L (<3 ng/mL), RBC folate levels <317 nmol/L RBC (<140 ng/mL), are considered to require follow-up because of potential folate deficiency.

Lab Tests Online gives a pretty good overview of their Vitamin B12 and Folate test, but they state:

Reference values are dependent on many factors, including patient age, gender, sample population, and test method, and numeric test results can have different meanings in different labs.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the specific reference range for your test(s). To see a sample lab report, click here. Please consult your doctor or the lab that performed the test(s) to obtain the reference range if you do not have the lab report.

If you have a question about the tests performed or your test results, you can submit a question using the Ask Us form. It will be answered by a laboratory scientist.

For more information on reference ranges, please read Reference Ranges and What They Mean.

Noting that the their Sample Report does not show ranges that seem congruent with your test values.

So, may want to ask you doc which method they used in the test and to explain the results to you. It may require the doc to call the lab, as this seems to get kind of complicated.

 

[Little Bluestem]

Wow, the Lab Tests Online reference ranges are very different than mine. Mine falls within the other two.

Clinical text books provide some information on reference ranges with the microbiologic assay, however those data were produced before folic acid fortification was introduced in the U.S. Therefore we don’t have post-fortification reference ranges for the U.S. population with the microbiologic assay.

This is why I dislike that mean ± SD methodology. If the reference range truly reflected optimum health, it wouldn't change when food was fortified. Assuming the average person is in optimum health is ridiculous.