# Use of immunoglobulin G homeostatic set point and recovery time in plasmapheresis donor safety monitoring: A retrospective observational cohort study

## TL;DR

This study examines how IgG levels in plasmapheresis donors change over time and identifies factors to predict when donors might have dangerously low IgG.

## Contribution

The study introduces the concept of IgG homeostatic set point and recovery time to improve donor safety monitoring in plasmapheresis.

## Key findings

- IgG levels decline with age and recover to baseline after 12 weeks of no donations.
- Donors with low initial IgG levels are most at risk of dangerously low IgG after frequent donations.
- Total protein measurements are not useful for monitoring donor safety.

## Abstract

Serum immunoglobulin G (IgG) and total protein are used to monitor plasmapheresis donor safety. However, there is a lack of information from large donor cohorts to determine the best use of these measurements.

We identified 230,144 plasmapheresis donors making their first donation between 1 July 2020 and 31 March 2024. IgG and total protein were measured prior to the first donation and then annually, following our donor safety monitoring protocol. We considered individuals who had not donated for 12 months to estimate intra‐individual biological variability of IgG. We compared four models to predict which donors would develop IgG < 6 g/L.

The IgG reference interval for the cohort was 7.67–15.6 g/L. IgG declines 5%–11% after the age of 45 years. The intra‐individual biological variability of IgG (5.2%) is small, indicating that there is homeostatic set point for individual IgG. IgG is reduced by plasmapheresis but recovers to recruitment level after 12 weeks. When plasma is donated every 2–3 weeks, mean IgG plateaus 1 g/L below recruitment concentration. IgG at recruitment is the best predictor of which donors will have IgG < 6 g/L after a year of donations. Total protein is a low‐value test in this context.

Plasmapheresis is safe and sustainable for almost every donor, at the 2‐weekly frequency allowed in Australia. The donors most likely to experience unacceptably low IgG are those with very low recruitment IgG levels. These donors could be recommended 12‐week intervals between donations or other donation types.