Falsely Elevated Corticotropin (ACTH) Levels Measured on Siemens Immulite 2000

Program: Abstracts - Orals, Featured Poster Presentations, and Posters
Session: SUN 50-71-HPA Axis
Clinical
Sunday, June 16, 2013: 1:45 PM-3:45 PM
Expo Halls ABC (Moscone Center)

Poster Board SUN-68
Luciana Pinto Brito*1, Guilherme Asmar Alencar2, Helena Panteliou Lima Valassi1, Marcia Ester Ferreira Paiva1, Maria Candida Barisson Villares Fragoso2, Berenice Bilharinho Mendonca1 and Marcelo Cidade Batista1
1Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo, São Paulo, Brazil, 2Hospital das Clínicas, Faculdade de Medicina da Universidade de São Paulo, Brazil
Introduction: Interference in immunoassays is a widely recognized problem, leading to spurious results and unnecessary additional tests.

Objective:Our aim was to report 5 patients with abnormally high ACTH levels measured on Siemens Immulite 2000 which were not consistent with the clinical diagnosis and to describe several commonly available laboratory-based approaches to investigate these spurious results.

Methods and Results: Samples from 2 patients (Cases 1 and 2) with bilateral adrenal macronodular hyperplasia during the dexamethasone suppression test (1mg PO every 6 h), 1 patient with panhypopituitarism (Case 3), 1 patient with an incidental adrenal mass (Case 4) and 1 obese patient (Case 5) were considered inadequate for clinical data when first measured on Siemens Immulite 2000 (normal range, up to 46 pg/mL). Plasma ACTH levels were repeated in the duplicate on Roche Modular E170 (normal range, 7 to 63 pg/mL). The detected levels were: Case 1 – 77.0 vs < 1.0 pg/mL; Case 2 – 12.0 vs < 1.0 pg/mL, Case 3 – 64.0 vs 3.8 pg/mL, Case 4 – 80.0 vs 3.7 pg/mL and Case 5 – 125.0 vs 18.0 pg/mL . In contrast, no differences were found when plasma ACTH was compared in 73 random samples measured on Immulite and E170 (68 vs 69 pg/mL, p= 0.8, by paired t-test). In Case 5, plasma ACTH levels varied from 80 to 200 pg/mL. Serial dilution of this sample up to 1:10 using pooled plasma with low ACTH levels showed a non-linear increase in ACTH recovery (226 to 508%), confirming the presence of an interfering substance. Anti-peroxidase antibodies were positive. Polyethylene glycol precipitation showed reduced ACTH recovery (8% in patient sample vs 68-87% in control samples), suggesting the presence of a high molecular weight interfering substance. Finally, the sample was incubated with plasma collected from a patient with primary adrenal insufficiency (273 pg/mL on Immulite) at 1:1 ratio. ACTH recovery after incubation was 138% (expected concentration, 207 pg/mL; measured concentration, 285 pg/mL on Immulite).

Conclusions: Our findings suggest the presence of a high molecular weight interfering substance on Immulite ACTH assay. This may be caused by heterophile antibodies or perhaps an anti-ACTH IgG. Further studies are under way to evaluate these possibilities.

(1) Sapin R., Interferences in immunoassays : mechanisms and outcomes  in endocrinology. Ann Endocrinol (Paris). 2008 Nov;             69(5):415-25. (2) Loh TP, Macro-Thyrotropin: A Case Report and Review of Literature. J Clin Endocrinol Metab, June 2012, 97(6):1823–1828

Nothing to Disclose: LPB, GAA, HPLV, MEFP, MCBVF, BBM, MCB

*Please take note of The Endocrine Society's News Embargo Policy at http://www.endo-society.org/endo2013/media.cfm