Stability of Fibroblast Growth Factor 23 in Human Plasma

Effect of vitamin D supplementation on circulating fibroblast growth factor-23 concentration in adults with prediabetes

BACKGROUND

Recent meta-analyses report that vitamin D supplementation increases blood fibroblast growth factor-23 (FGF23) level.

OBJECTIVES

To determine the effect of 4000 IU/day of vitamin D₃ for 12 months on circulating FGF23 levels. We also examined the association of the achieved 25-hydroxyvitamin D level [25(OH)D] with the FGF23 level at 12 months and with 12-month changes in FGF23.

METHODS

An ancillary analysis among adults 70 years and older with prediabetes who participated in a trial comparing vitamin D₃ 4000 IU/day with placebo. Plasma intact FGF23 and serum 25(OH)D were measured at baseline and month 12 (M12).

RESULTS

Characteristics of the 52 participants (vitamin D₃ n = 28; placebo n = 24) did not differ significantly aside from more women than men in the vitamin D₃ group. Mean ± SD age was 73.8 ± 3.7 years, BMI 31.3 ± 4.2 kg/m2, and glomerular filtration rate (GFR) 76.3 ± 11.8 mL/min/1.73m² Baseline serum 25(OH)D level was 33.4 ± 10.8 ng/mL and increased at M12 to 54.9 ± 14.8 ng/mL in the vitamin D₃ group versus 33.4 ± 14.9 in the placebo (p < 0.001). At baseline, GFR was inversely associated with FGF23 (r = - 0.349, p = 0.011). Change in FGF23 level at M12 did not differ significantly between vitamin D₃ and placebo. In all participants combined, the achieved serum 25(OH)D level at M12 was not significantly associated with the M12 plasma FGF23 or the M12 change in FGF23.

CONCLUSION

In obese older adults with sufficient vitamin D status and normal renal function, vitamin D₃ 4000 IU/day for 12 months did not significantly alter plasma intact FGF23 levels.

CLINICALTRIALS

gov NCT 01,942,694, registered 9/16/2013.

The Measurement and Interpretation of Fibroblast Growth Factor 23 (FGF23) Concentrations

Two decades after the discovery of the hormone FGF23, we know more about phosphate homeostasis as it turned out that FGF23 is the central hormone that regulates this. Hereditary hypophosphatemic rickets and tumor-induced osteomalacia could by then be explained, by autonomous FGF23 production, and the nephrology field was excited by this new marker as it turned out to be independently associated with mortality in people treated by hemodialysis. This led to the development of several immunoassays to be able to measure FGF23 in blood. In the past years we learned that FGF23 is a rather stable peptide, the precision of the assays is acceptable but assays are not standardized and therefore not comparable. This means that reference values and cutoff values need to be assay specific. For several assays reference values have been established and gender and age did not seem of high importance. The phosphate content of the diet, which can be culturally dependent, however, should be taken into account when interpreting results, but to what extent is not totally clear. Currently, clinical application of the immunoassays is established in the diagnosis of hereditary hypophosphatemic rickets and diagnosis and follow-up of tumor-induced osteomalacia. Definite conclusions on the usefulness of the FGF23 measurement in people with CKD either as a marker for risk prediction or a as target for treatment remains to be determined. The latter applications would require dedicated prospective clinical trials, which may take years, before providing answers. To improve the standardization of the FGF23 assays and to shed light on the biological functions that fragments might have we might aim for an LC-MS/MS-based method to quantify both intact and fragmented FGF23. In this literature review we will summarize the current knowledge on the physiological role of FGF23, its quantification, and the clinical usefulness of its determination.

Determination of iFGF23 Upper Reference Limits (URL) in healthy pediatric population, for its better correct use

BACKGROUND

The measurement of Fibroblast growth factor 23 (FGF23) may be useful in the diagnosis and management of abnormal phosphate metabolism in both patients with preserved renal function or with chronic kidney disease (CKD). FGF-23 tests differ considerably by molecule assayed (iFGF23 or cFGF23), analytical performance and reference ranges. We establish iFGF23 Upper Reference Limits (URL) in apparently healthy pediatric individuals using automated immunochemiluminescent assay.

METHODS

We measured the levels of plasma iFGF23 from 115 samples from apparently healthy pediatric subjects [59 (51.3%) individuals were male; median age 10 years (range 1-18)] included in an observational study conducted at Policlinico University Hospital of Bari. The method used for the iFGF23 assay was immunochemiluminescent sandwich assay developed by DiaSorin on the Liaison XL platform. Statistical calculation of 95% reference interval, right-sided (CLSI C28-A3) and verification of age and sex covariables was performed for the calculation of the URL.

RESULTS

The URL concentration of iFGF23 was 61.21 pg/mL (58.63 to 63.71, 90% CI). No significant differences were found between the median concentrations of iFGF23 differentiated by sex and age.

CONCLUSIONS

The dosage of iFGF23 is important both for the differential diagnosis of the various forms of rickets, and for the subsequent monitoring of the effectiveness of drug treatment. We have established the URL for the iFGF23 Liaison test in apparently healthy pediatric subjects. The availability of iFGF23 pediatric reference values will allow a better clinical use of the test.

The Effects of Storage Time and Repeated Freeze-Thaw Cycles on Intact Fibroblast Growth Factor 23 Levels

Background: Fibroblast growth factor 23 (FGF23) has become increasingly important in chronic kidney diseases (CKDs), cardiovascular calcification, and metabolic bone diseases. Fresh or stored blood samples are widely used for the FGF23 assay. Clarifying the factors influencing the FGF23 assay can help to quantify FGF23 more accurately. This study explored the effects of low-temperature storage time and repeated freeze-thaw cycles on the measurement of serum intact FGF23 (iFGF23). Materials and Methods: We selected 60 serum samples from patients with CKD stages 3-5 and hemodialysis patients. An enzyme-linked immunosorbent assay was used to measure the changes in serum iFGF23 levels after 6 years of storage at -80°C. In total, 18 fresh serum samples were frozen and thawed for 0, 1, 3, and 5 cycles to explore the effects of repeated freeze-thaw cycles on serum iFGF23 levels. Results: Median serum iFGF23 concentrations were 252.17 (interquartile range [IQR] 113.82-592.38) pg/mL and 203.85 (IQR 64.76-545.39) pg/mL before and after 6 years. There were no significant differences between them. However, we found a downward trend of 48% in the samples close to the normal level of iFGF23 (<150.34 pg/mL) after 6 years of storage (p = 0.160). In addition, the iFGF23 levels of samples frozen and thawed for 0, 1, 3, and 5 cycles were 278.41 ± 39.51 (mean ± standard deviation) pg/mL, 262.84 ± 38.42 pg/mL, 252.97 ± 34.65 pg/mL and 250.49 ± 37.12 pg/mL, respectively. A slight downward trend in iFGF23 levels was observed with increasing freeze-thaw times; however, no significant differences were found among different freeze-thaw cycles. Conclusion: Serum iFGF23 levels remained stable after storage at -80°C for 6 years. In addition, five freeze-thaw cycles had no significant effects on serum iFGF23 levels.

Fibroblast growth factor 23: are we ready to use it in clinical practice?

Patients with chronic kidney disease (CKD) have a greatly enhanced risk of cardiovascular morbidity and mortality. Over the past decade it has come clear that a disturbed calcium-phosphate metabolism, with Fibroblast Growth Factor-23 as a key hormone, is partly accountable for this enhanced risk. Numerous studies have been performed unravelling FGF23s actions and its association with clinical conditions. As FGF23 is strongly associated with adverse outcome it may be a promising biomarker for risk prediction or, even more important, targeting FGF23 may be a strategy to improve patient outcome. This review elaborates on the clinical usefulness of FGF23 measurement. Firstly it discusses the reliability of the FGF23 measurement. Secondly, it evaluates whether FGF23 measurement may lead to improved patient risk classification. Finally, and possibly most importantly, this review evaluates if lowering of FGF23 should be a target for therapy. For this, the review discusses the current evidence indicating that FGF23 may be in the causal pathway to cardiovascular pathology, provides an overview of strategies to lower FGF23 levels and discusses the current evidence concerning the benefit of lowering FGF23.

Pre-analytical stability of FGF23 with the contemporary immunoassays

BACKGROUND

Several publications have reported on the pre-analytical stability of fibroblast growth factor 23 (FGF23) and some recommend coating blood collecting tubes with protease inhibitors, in order to prevent degradation. These recommendations are based on observations for a first generation assay for the measurement of intact FGF23. However, if this also applies for the contemporary immunoassays, and at what stage of pre-analysis, is unknown.

METHODS

We reviewed data from these previous reports on the issue of FGF23 stability and complemented these findings with data from novel experiments.

RESULTS

We concluded that the contemporary intact FGF23 assays by Immutopics, Kainos, Millipore and DiaSorin do not suffer from immediate loss of FGF23 signal and do not require blood withdrawal in protease inhibitor-coated collecting tubes. Nevertheless, FGF23 concentrations do decline when centrifugation is delayed up to 8 h and prompt centrifugation is therefore advised.

Biological variation of intact fibroblast growth factor 23 measured on a fully automated chemiluminescent platform

Background

Fibroblast growth factor 23 (FGF23), a potent regulator of phosphate and vitamin D metabolism, is a new biomarker of kidney, bone and cardiovascular disorders. The aim of this study was to assess the biological variation of intact fibroblast growth factor 23 (iFGF23).

Methods

The within-subject (CVI) and between-subject (CVG) biological variations were assessed in 14 healthy volunteers in a six-week protocol (seven samples). Imprecision (CVA) was assessed by duplicate measurements and the EP15-A2 protocol. Intact FGF23 was measured using a fully automated chemiluminescent assay (Liaison XL, DiaSorin S.p.A., Saluggia, Italy). Two methods with different sensitivities to non-Gaussian distribution were used to estimate the CVI, SD ANOVA and CV ANOVA methods. We calculated the index of individuality (II) and reference change values.

Results

Depending on the statistical method used, the CVI and CVA were 14.2 and 3.7% (SD ANOVA) or 12.5 and 3.9% (CV ANOVA), respectively. The corresponding reference change values were 40.5 and 36.4%, respectively. The CVG was 13.4% (SD ANOVA was the only option), and the total imprecision (EP15-A2) was less than 7%.

Conclusions

The measurement of iFGF23 demonstrated a CVA less than 4% during the experimental estimation of biological variation. The total imprecision was less than 7% in the EP15-A2 experiment. The CVI values of iFGF23 in healthy persons were 14.2 (SD ANOVA) and 12.5% (CV ANOVA), respectively. The CVG was 13.4%, and the resulting index of individuality was 1.06. The reference change value was less than 41%. The availability of this automated assay for iFGF23 with well-characterized biological variation data delivers opportunities for improved availability and application of this assay clinically.

Fibroblast Growth Factor 23 Associates with Death in Critically Ill Patients

BACKGROUND AND OBJECTIVES

Dysregulated mineral metabolism is a common and potentially maladaptive feature of critical illness, especially in patients with AKI, but its association with death has not been comprehensively investigated. We sought to determine whether elevated plasma levels of the osteocyte-derived, vitamin D-regulating hormone, fibroblast growth factor 23 (FGF23), are prospectively associated with death in critically ill patients with AKI requiring RRT, and in a general cohort of critically ill patients with and without AKI.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We measured plasma FGF23 and other mineral metabolite levels in two cohorts of critically ill patients (n=1527). We included 817 patients with AKI requiring RRT who enrolled in the ARF Trial Network (ATN) study, and 710 patients with and without AKI who enrolled in the Validating Acute Lung Injury biomarkers for Diagnosis (VALID) study. We hypothesized that higher FGF23 levels at enrollment are independently associated with higher 60-day mortality.

RESULTS

In the ATN study, patients in the highest compared with lowest quartiles of C-terminal (cFGF23) and intact FGF23 (iFGF23) had 3.84 (95% confidence interval, 2.31 to 6.41) and 2.08 (95% confidence interval, 1.03 to 4.21) fold higher odds of death, respectively, after adjustment for demographics, comorbidities, and severity of illness. In contrast, plasma/serum levels of parathyroid hormone, vitamin D metabolites, calcium, and phosphate were not associated with 60-day mortality. In the VALID study, patients in the highest compared with lowest quartiles of cFGF23 and iFGF23 had 3.52 (95% confidence interval, 1.96 to 6.33) and 1.93 (95% confidence interval, 1.12 to 3.33) fold higher adjusted odds of death.

CONCLUSIONS

Higher FGF23 levels are independently associated with greater mortality in critically ill patients.