Direct Answer
There is no validated conversion factor between RU/mL (relative units) and pg/mL (picograms per milliliter) for FGF-23 measurements, as these represent fundamentally different assay methodologies that cannot be directly converted.
Why Conversion Is Not Possible
The value you have (83 RU/mL, reference range 44-215 RU/mL) appears to be from a C-terminal FGF23 (cFGF23) assay, which measures both intact FGF23 and C-terminal fragments in relative units. The pg/mL measurement refers to intact FGF23 (iFGF23) assays, which measure only the biologically active intact hormone 1.
Key Differences Between Assays:
- C-terminal assays (RU/mL): Measure total FGF23 including fragments; less specific for diagnosis
- Intact assays (pg/mL): Measure only active hormone; superior diagnostic accuracy
These are not interchangeable measurements and represent different molecular species.
Clinical Interpretation of Your Result
Your FGF-23 value of 83 RU/mL is within the laboratory reference range (44-215 RU/mL), but this does NOT exclude FGF23-mediated hypophosphatemia if you have concurrent hypophosphatemia.
Critical Diagnostic Pitfall:
Laboratory "normal" ranges for FGF23 were established in normophosphatemic individuals and can be misleading in hypophosphatemic patients 1. Research demonstrates that using the laboratory upper limit of normal for cFGF23 (180 RU/mL) would result in misdiagnosis in more than half of patients with FGF23-mediated hypophosphatemia 1.
Diagnostic Cut-Off Values
If you need to evaluate for FGF23-mediated hypophosphatemia:
For Intact FGF23 (pg/mL):
- Cut-off: 27 pg/mL is 100% sensitive and specific for distinguishing FGF23-mediated from FGF-independent hypophosphatemia 1
- Even "low-normal" intact FGF23 levels suggest FGF23 excess when hypophosphatemia is present 2
For C-terminal FGF23 (RU/mL):
- Cut-off: 90 RU/mL is 100% sensitive and specific for tumor-induced osteomalacia specifically 1
- Range 45-90 RU/mL shows overlap between genetic FGF23 excess and FGF23-independent causes, making cFGF23 inferior to iFGF23 for diagnosis 1
Your value of 83 RU/mL falls in this problematic overlap zone where interpretation requires clinical context.
Practical Recommendation
If you are evaluating for FGF23-mediated hypophosphatemia (such as XLH or tumor-induced osteomalacia), request an intact FGF23 assay measured in pg/mL rather than attempting to convert your C-terminal result 2, 1. The intact assay provides superior diagnostic accuracy and has validated cut-off values for clinical decision-making.