Hypersomatotropism [HS] is the underlying cause of diabetes mellitus in a substantial number of diabetic cats. Serum insulin-like growth factor-1 [IGF-1] measurement is currently the test of choice, with concentrations greater than 1000 ng/ml having a 95% positive predictive value. Therefore, all commercial laboratories currently use this value as a cut-off to indicate likely HS presence. However, endogenous insulin availability affects IGF-1 synthesis, thereby possibly reducing test sensitivity, especially in newly insulin-treated diabetic acromegalics.

This study's two main questions were: 1. How many newly diagnosed and treated diabetic cats demonstrate serum IGF-1 initially not suggestive to subsequently become suggestive of HS using this cut-off (1000 ng/ml)? 2. How strong is the correlation between endogenous insulin and IGF-1 concentrations in untreated diabetic cats?

Serial blood samples of diabetic cats were prospectively recruited from first opinion UK veterinary practices within 180 days of initiating insulin treatment. Serum IGF-1 and basal serum endogenous insulin were evaluated using a validated and commercially offered RIA and ELISA, respectively; the latter in untreated diabetic cats only. Mann Whitney test was used to compare groups and Spearman rank correlation coefficient to assess correlation between endogenous insulin and IGF-1; p<0.05 was considered significant. Serial blood samples of 219 cats were recruited (two samples: 103 cats; three: 55; four: 44; ≥five: 17). Sixty-two (28.3%) cats had at least one IGF-1 measurement >1000 ng/ml (median 1576 ng/ml, range 1001–>2000); a median of 0.6 units/kg/injection insulin (range 0–2.4) was administered. Of the cats with IGF-1 >1000 ng/ml, 20 (9.1%) initially showed IGF-1 <1000 ng/ml; therefore, the attending clinician could have discarded the possibility of HS in these patients. Median subsequent IGF-1 increase was 594 ng/ml (range 71–1495), having initially received a median of 73 days of insulin treatment (range 25–154). Basal endogenous insulin in untreated diabetic cats with IGF-1 <1000 ng/ml (n=106; median 29.0 ng/l, range 9.2–791) was significantly lower than in those with IGF-1 >1000 ng/ml (n=15; median 64.2 ng/l, range 9.2–490; p=0.024). A moderate positive correlation (r_s=0.42, p<0.0001) was detected between endogenous insulin and IGF-1 in untreated cats. In this study, approximately 1 in 10 newly diagnosed diabetic cats with an IGF-1 suggestive of underlying HS, based on the currently advocated cut-off, will initially show a negative value using this cut-off. Lower endogenous insulin, which moderately correlates with IGF-1, and/or suboptimal current cut-off value advice, could be contributing factors.

Disclosures

No disclosures to report.