title: Commercial insulin immunoassays fail to detect commonly prescribed insulin analogues.
abstract: OBJECTIVE
  Blood insulin and C-peptide are key investigations in the differential diagnosis of hypoglycaemia. Analogues of insulin have modified primary-sequences compared to native human insulin, as such may not cross react with insulin assays. This has important implications in detecting surreptitious or malicious insulin administration. The aim of this study is to assess the cross-reactivity of all insulins currently listed in the British National Formulary (BNF65, 2013) in clinical insulin assays currently used in UK clinical laboratories.
  
  
  METHODS
  Sample sets were prepared for all 15 exogenous insulin classes listed in the BNF, at concentrations of 1000 pmol/L and 300 pmol/L, using pooled human serum. Samples were sent blinded to 5 participating analytical laboratories to cover analysis on the 10 major clinical insulin assays used in the UK.
  
  
  RESULTS
  The ability of insulin assays to detect exogenous insulin preparations was highly variable and ranged from 0% to >140% for a single exogenous insulin. Four assays were highly specific for the human insulin sequence and had no cross-reactivity with any synthetic analogue insulin. Two detected all insulin types (human sequence, animal and synthetic analogue), with the remaining having variable cross-reactivity.
  
  
  CONCLUSIONS
  The cross-reactivity of the 15 exogenous insulin preparations is highly variable in the assays used in clinical laboratories around the UK. It is important that laboratories and clinicians are aware of the limitations of their local assays to avoid missing the important diagnosis of hypoglycaemia secondary to excessive exogenous insulin. Where necessary, samples should be referred to specialist centres for insulin analysis and ideally by a validated and fully-quantitative mass spectrometry-based method.
authors.affiliation: Blood Sciences, Royal Devon and Exeter Foundation Trust, UK.
authors.forename: C
authors.initials: C
authors.lastname: Parfitt
authors.affiliation: Department of Clinical Biochemistry, Addenbrooke's Hospital, UK; University of Cambridge Metabolic Research Laboratories, UK.
authors.forename: D
authors.initials: D
authors.lastname: Church
authors.affiliation: Department of Laboratory Medicine, University Hospital Southampton, UK.
authors.forename: A
authors.initials: A
authors.lastname: Armston
authors.affiliation: Viapath, Department of Clinical Biochemistry, King's College Hospital, London, UK.
authors.forename: L
authors.initials: L
authors.lastname: Couchman
authors.affiliation: Department of Medical Biochemistry & Immunology, University Hospital of Wales, UK.
authors.forename: C
authors.initials: C
authors.lastname: Evans
authors.affiliation: Surrey Pathology Services, Royal Surrey County Hospital, UK.
authors.forename: G
authors.initials: G
authors.lastname: Wark
authors.affiliation: Blood Sciences, Royal Devon and Exeter Foundation Trust, UK; NIHR Exeter Clinical Research Facility, University of Exeter, UK. Electronic address: Timothy.mcdonald@nhs.net.
authors.forename: T J
authors.initials: TJ
authors.lastname: McDonald
mesh_terms: D000911:Antibodies, Monoclonal / Q000737:chemistry; D015415:Biomarkers / Q000097:blood; D002096:C-Peptide / Q000097:blood*; D003429:Cross Reactions; D006801:Humans; D007003:Hypoglycemia / Q000097:blood* / Q000139:chemically induced / Q000175:diagnosis; D007118:Immunoassay / Q000592:standards*; D007328:Insulin / Q000009:adverse effects / Q000031:analogs & derivatives / Q000097:blood*; D055553:Prescription Drugs / Q000008:administration & dosage / Q000378:metabolism*; D012680:Sensitivity and Specificity
chemical_list: D000911:Antibodies, Monoclonal; D015415:Biomarkers; D002096:C-Peptide; D007328:Insulin; D055553:Prescription Drugs
keywords: Analogues; Cross-reactivity; Hypoglycaemia; Immunoassay; Insulin