SummaryInsulin autoimmune syndrome (IAS) is a rare cause of hyperinsulinemic hypoglycaemia, which is known to occur in association with the use of sulfhydryl-containing drugs and autoimmune disorders. We describe a patient with hitherto an unreported association of IAS with ankylosing spondylitis. We have also performed and described a simplified method of polyethylene glycol (PEG) precipitation of an insulin bound antibody in the serum.Learning pointsIAS should be considered in differential diagnosis of endogenous hyperinsulinemic hypoglycaemia.Ankylosing spondylitis can be associated with IAS apart from several other autoimmune diseases.Very high serum insulin levels (100–10 000 μU/ml) are frequently seen in IAS.When faced with very high serum insulin before suspecting insulinoma, it is advisable that PEG precipitation of serum be done to identify antibody bound insulin.A clinical suspicion of IAS can avoid expensive imaging and unnecessary surgery in affected patients.

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