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Comparison of second and third-generation parathyroid hormone assays at a tertiary hospital in South Africa
Nokuthula Nhlapo1, Doreen Jacob1,2, Siyabonga Khoza1,2
1Department of Chemical Pathology, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa.
Background:
Parathyroid hormone (PTH) measurement is key for diagnosing parathyroid disorders, and for management of chronic kidney disease. Available PTH assays include second (intact PTH) and third (PTH 1-84) generations. Data comparing interchangeable use are insufficient.
Objective:
The objective of this study was to compare intact and 1-84 PTH assays to determine the difference in analytical performance and impact on clinical interpretation.
Methods:
A method comparison was done on residual samples with PTH requests (06 April 2022 - 21 September 2022) from a tertiary hospital in South Africa. Parathyroid hormone was measured using both intact PTH and 1-84 PTH assays. Clinical performance was compared in the diagnosis of hypo- and hyperparathyroidism, and in pre-dialysis and dialysis chronic kidney disease patients.
Results:
Among 481 samples, intact PTH had a higher median concentration than PTH 1-84 (9.85 pmol/L vs. 8.51 pmol/L, p < 0.0001), but the two showed good correlation (r = 0.994, p < 0.0001). Regression analysis revealed systematic (intercept = 0.887 pmol/L [95% confidence interval: 0.788 - 1.005]) and proportional differences (slope = 0.713 pmol/L, [95% confidence interval: 0.703 - 0.723]), with increased deviations at higher concentrations. The average bias was 18.5%, exceeding allowable limits. Among the 276 patients (170 women, 106 men, age range: 18-89 years) included in the clinical study, interpretation was unchanged.
Conclusion:
A bias was observed between the PTH assays, indicating that they should not be used interchangeably. However, no changes in clinical interpretation were observed when one assay was used over the other.
What This Study Adds:
The study confirms the recommendation by Kidney Disease: Improving Global Outcomes for the use of assay-specific upper limit of normal instead of generic cut-off in dialysis patients. This study further highlights the need for standardisation of PTH assays.
