Comparison of Liaison N-tact PTH (Diasorin) and N-tact PTH SP IRMA (Diasorin) in hemodialyzed patients

The path to the standardization of PTH: Is this a realistic possibility? a position paper of the IFCC C-BM

Parathyroid hormone (PTH) determination is of greatest importance for patients suffering from parathyroid gland disorders and for the follow-up of bone turnover in patients suffering from chronic kidney disease (CKD). Two generations of PTH assays are simultaneously present on the market for PTH quantification. As these assays are not yet standardized, this results in a significant level of confusion in the care of CKD patients. One key objective of the IFCC Committee for Bone Metabolism is to improve this situation. In this position paper, we will highlight the current state of PTH testing and propose a pathway to ultimately overcome issues resulting from PTH assay variability.

False positive PTH results: An easy strategy to test and detect analytical interferences in routine practice

BACKGROUND

As other immunoassays, PTH determination is not free from interferences. Indeed, natural antibodies like heterophile antibodies (HAMA) and rheumatoid factor (RF) can induce falsely elevated results, leading to misdiagnosis and expensive unnecessary explorations. However, in routine practice, these interferences are not always obvious to detect.

METHODS

On 2084 PTH samples, we applied a validation strategy in four steps to screen for HAMA and rheumatoid factor interferences.

RESULTS

36% of our samples presented an elevated PTH. We found a clinically plausible reason for 91% of them. The remaining 63 suspicious samples were treated with HBT and 40% of them were found to be HAMA positive. RF determination was performed on the HAMA-negative samples and RF was positive in 21 of them. They were then treated with RF-Absorbent. Nine of these 21 samples presented RF interference.

CONCLUSION

Applying this strategy in our routine validation, we managed to avoid spuriously elevated PTH results, which could have caused medical errors as well as unnecessary cost-effective extra-investigations.

Analytical validation of the new version of the Liaison N-Tact PTH assay

We performed analytical validation of the new version of the Liaison N-Tact PTH (parathormone) assay according to NCCLS guidelines and compared this new generation of reagent with the Roche Elecsys PTH assay. We showed that this new version is a sensitive and precise method with good recovery. Significant carryover was observed, but with limited clinical implications. We demonstrated that the new version of the Liaison PTH is in reasonably good agreement with the Roche Elecsys PTH assay, and as we observed no differences in a hemodialyzed population, moving from one method to the other should not affect the daily follow-up of patients. However, one should be cautious with the highest values (>500 pg/mL). We established reference intervals of 12-54 pg/mL for the Liaison and 14-52 pg/mL for the Elecsys assay.