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|Patient Preparation||Overnight fasting before specimen collection is required.|
|Collection Instructions||Pre-chill lavender top in ice prior to ,collection. Transport sample to lab on ice immediately after collection.,|
|Container type||Lavender top (on ice)|
|Amount to Collect||3 mL blood|
|Sample type||EDTA Plasma|
|Preferred volume||1.5 mL plasma|
|Min. Volume||0.7 mL plasma|
|UCSF Rejection Criteria||Grossly hemolyzed or lipemic samples.|
|Processing notes||Use refrigerated centrifge to spin sample down. Aliquot 1,5 mL plasma into plastic vial and freeze at -20C. Transport frozen on dry ice.|
|Ref Lab Rejection Criteria||Sample received at room temperature, gross hemolysis or lipenmia|
|Normal range||3 - 20 pmol/L|
|Stability||Refrigerated 1 week, frozen at -20C 2 weeks.|
|Turn around times||4-7 days|
|Additional information||Normal individuals will have proinsulin concentrations below the upper limit of the normal fasting reference range (20 pmol/L) when hypoglycemic (blood glucose <45-60 mg/dL). Conversely, most (>80%) insulinoma patients will have proinsulin concentrations above the upper limit of the reference range. The sensitivity and specificity for a diagnosis of insulinoma during hypoglycemia are approximately 75% and near 100%, respectively, at the 20 pmol/L cutoff. A higher sensitivity (>95%) can be achieved using a 5 pmol/L cutoff, and this is the cutoff recommended by the Mayo Clinic's highly experienced hypoglycemia team to avoid missing cases. However, the lower cutoff results in a reduced specificity (approximately 40%), emphasizing the need for a combination of different tests to assure accurate biochemical diagnosis.
Patients with PC1/3 deficiency have low, or sometimes undetectable, insulin levels and substantially elevated proinsulin levels, exceeding the upper limit of the reference range substantially in the fasting state and rising even higher after food intake. Many other hormonal abnormalities are also present, including cortisol deficiency (because of lack of processing of pro-opiomelanocortin to adrenocorticotropic hormone and other peptides), infertility and, often, morbid obesity.
This assay demonstrates no cross-reactivity with insulin or C-peptide.
A polyclonal capture antibody-conjugated bead recognizing human insulin is incubated with standards, controls and patient samples, capturing insulin and proinsulin, but not free C-peptide. Following washing, a polyclonal acridinium ester-labeled antiserum that recognizes C-peptide is added, binding to captured proinsulin, but not to captured insulin. After overnight incubation, the bead is washed, and flash-chemiluminescence is triggered and measured. The chemiluminescence signal is proportional to the concentration of proinsulin in the sample. (Kao PC, Taylor RT, Service FG: Proinsulin by novel ICMA-diagnostic implication for insulinoma. 75th Annual Meeting, Endocrine Society, #740, p 235, 1993; Kao PC, Taylor RT, Service FG: Proinsulin by immunochemiluminometric assay for the diagnosis of insulinoma. J Clin Endocrinol Metab 1994;78:1048-1051)
|Last Updated||7/24/2012 9:51:09 AM|