Endocrine Abstracts

Background: The suitability of serum insulin-like growth factor 1 (IGF-1) reference ranges (RR) used locally (manufacturer-derived), were raised as a clinical concern. IGF-1 levels above the RR were reported in patients displaying no clinical signs or symptoms of acromegaly, and in whom growth hormone suppression tests were negative. No analytical issue was evident, and quality performance indicators were satisfactory.

Methods: A review of all serum IGF-1 results (Siemens IMMULITE® 2000 analyser) from July 2018-2019 was undertaken, with a total of 2162 (1151 male) patients included. Results were reviewed according to age- and gender-related RR, and assigned to one of three groups, low, normal, or high. All high IGF-1 results were reviewed alongside clinical information to determine the rate of true and false positives. Comparative stability of serum IGF-1 was also assessed at 4°C and room temperature (RT).

Results: Over the period reviewed, 8.9% of all serum IGF-1 results in adult patients (age >18) were classed as high, and 4.6% as low. For paediatric patients, there was little difference between high and low groups (11.2% and 9.6%, respectively). Further investigation into the adult ‘high’ group showed that the false positive rate was 2.53%, which may be expected from a Gaussian distribution where 95% of the population fit within the RR. Serum IGF-1 (n=24, range: 7.5-91 nmol/l) showed a mean change from day 0 to day 1 of 10.5% (SD=7.5%) for samples stored at RT, compared with a mean change of 3.5% (SD=3.0%) for samples stored at 4°C.

Conclusion: The review found no evidence that the serum IGF-1 RR in use are ineffective. However, serum IGF-1 concentrations were affected by pre-analytical storage at RT, a consequence of the introduction of a new automated track system. Changes made based on this review have improved clinicians’ confidence in interpreting IGF-1 results.