Preanalytical Phase of Soluble Receptor of Transferrin

Dear Editor,

I greatly appreciated the valuable data presented in the paper of Mayr et al. [[7]]. The comparison between haematological values in young sedentary people and age-matched athletes is precious for evaluating the possible modifications induced by continuous and consistent training and I quoted the data, already preliminary presented in a meeting organized by myself, in a book of sports haematology [[2]]. I would like to point out the argument of preanalytical phase of sTfR: the conclusive statement of the paper concerns the possible use of EDTA-derived plasma for measuring soluble transferrin receptor (sTfR), although limited to the study purposes [[7]].

sTfR should be measured in serum, because its concentration is higher in EDTA-plasma, rising progressively with storage time before centrifugation [[4]]. EDTA has some effects on the concentrations, immunologically measured, of cytokines and related molecules [[1], [6]]. In the paper of Mayr et al. the overestimation is confirmed in the comparison between serum and EDTA-plasma by the intercept of 0.4 for the whole range of measure, as testified by a slope of 1.01 [[7]].

The use of sTfR in sports medicine is precious, because this parameter is sensitive and specific for identifying functional impairment of iron metabolism and for modifying diet and iron intake, and also for suggesting further investigations about iron losses. We recommended sTfR, in association with reticulocytes, for accurately monitoring top-level athletes during a competitive season [[3]]. The sTfR was also introduced in laboratory procedures for suspecting abuse of erythropoietin (rHuEpo). In fact, sTfR is correlated with rHuEpo concentrations and should be used as indirect signal for selecting athletes for urine isoelectrofocusing [[8]]. The serum parameter is part of ONhes model for screening the athletes for rHuEpo abuse approved by some sports federations [[5]]. The introduction of sTfR in these procedures was linked to its high sensitivity, but some possible pitfalls must be taken into account. Besides the use of serum and plasma, that is consistent, because the difference is about half of the range considered as reference in general population by the producer of immunonephelometric method used in [[7]], and the absence, in my knowledge, of published biological variability for calculating critical difference, the lack of an international standard for sTfR impairs the comparability of measurements obtained by different methods. For example, the immunonephelometric method from Dade Behring (Marburg, Germany) is standardized against the sTfR monomeric truncated form isolated from human sera pool, whereas the IDeA turbidimetric method (Orion, Turku, Finland) is standardized against human dimeric sTfR, and the turbidimetric method from Biokit (Barcelona, Spain) against a human-derived protein, without the specification of monomeric or dimeric. Biokit allows the use of serum and plasma (heparin, EDTA, citrate), but reference values are reported only for serum, whereas Dade Behring allows serum and heparinized plasma (EDTA-derived plasma is not included), but reference values are reported only for serum. The Orion method permits the use of serum and EDTA, citrated, and heparinized plasma, with the warning only for different dilution of plasma against serum, but reference values are reported only for serum [[9]]. The producers should improve the informations for users.

The lack of standardization induces different reference ranges and consistent differences also in male athletes' population, as demonstrated in a series of 76 soccer players competing in teams of First, Second, and Third Divisions of Italian Championships, and of 29 alpine skiers of the Italian National Team.

The blood drawings were performed before the start of training and competitive season; sTfR was measured by means of Biokit and IDeA Orion turbidimetric assays on Aeroset (Abbott, Chicago, USA) (Fig. [1]). The median of sTfR concentrations of soccer players measured by the first method is slightly higher than the upper limit of the range of measures of the second one. The median of sTfR concentrations of skiers measured by the first method is near to the same limit of the second one. The differences between the two groups of athletes are qualitatively similar (for both the statistical significance was present; p < 0.05), but quantitatively different.

Fig. 1 sTfR in 76 professional soccer players and in 29 professional alpine skiers. A: values of soccer players measured by turbidimetric Biokit method (median: 1.64 mg/l; 2.5 - 97.5th percentiles: 1.06 - 2.31). B: values of skiers measured by turbidimetric Biokit method (median: 1.41 mg/l; 2.5 - 97.5th percentiles: 1.10 - 2.03); +: outlier (> 97.5th percentile). C: values of soccer players measured by turbidimetric IDeA method (median: 1.21 mg/l; 2.5 - 97.5th percentiles: 0.85 - 1.62). D: values of skiers measured by turbidimetric IDeA method (median: 1.07 mg/l; 2.5 - 97.5th percentiles: 0.83 - 1.46).

The presence of some differences among methods can be emphasized by the use of different biological materials, inducing confusion, possibly influencing the rationale and useful application of the parameter.

The use of sTfR should be recommended in sports medicine, but preanalytical problems should be known and reported, detailing the material and methods used and also the reference range calculated on sedentary people or a specific population used for interpretations of results in athletes.

References

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