Labs May Vary in iPTH Measurements

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NATIONAL HARBOR, Md.—Laboratories can differ in their measurements of intact parathyroid hormone (iPTH) levels, resulting in varying treatment approaches for hemodialysis (HD) patients depending on the iPTH assay used, researchers reported at the National Kidney Foundation Spring Clinical Meetings.

In a poster presentation, the researchers, Lynne Gafford, NP, and Hassan Gehmi, MD, of Henry Ford Health System in Detroit, said they observed in their patient population a large discrepancy in iPTH values after switching from an outside laboratory analyzing their iPTH (Lab 1) to the laboratory already analyzing all the rest of their hemodialysis patients' blood (Lab 2). They also observed a large increase in the amount of vitamin D analogue prescribed to treated secondary hyperparathyroidism (SHPT).

The investigators reviewed monthly iPTH values and vitamin D dose average per month for 99 in-center HD patients, comparing six months from Lab 1 and then the following six months from Lab 2. At the end of one year, they collected blood and analyzed iPTH from the same 99 patients comparing Lab 1 and Lab 2. A representative sample of 31 patients had the iPTH analyzed at a third independent laboratory (Lab 3).

The researchers found significant differences between Lab 1 and Lab 2 at the end of one year and significant differences between the labs in mean and median iPTH values for the two time periods. When the investigators tabulated values for the 31 blood samples analyzed at Lab 3 side-by-side with Lab 1 values, they found similar concordance. 

Many factors have been identified as contributors to the variability observed in iPTH results, and these range from specimen handling to the choice of antibody used in the assay, the researchers stated.

"Improving the comparability in the various assays may need to involve measures to standardize PTH reporting by different methods, the adoption of assay-specific clinical decision guidelines, and potentially re-examining existing evidence and new requirements of a valid and reproducible PTH assay," the authors noted.

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