Glycosylated hemoglobin (HbA1) in a case of insulinoma

Glycated Hemoglobin in Insulinoma

OBJECTIVE

To determine whether preoperative levels of glycated hemoglobin (GHb) are lower in patients with functioning insulinoma and, if so, whether a distinct separation of GHb values from those in control subjects might serve for diagnosis.

METHODS

We examined preoperative GHb in consecutive patients (who had this measurement done) with surgically confirmed insulinoma for the period 1983 (when the current method became available for routine use) through 1996. Hemoglobin A(1) (HbA(1)) was measured by the Isolab Glyc-Affin Test System (normal range, 4 to 7%). We studied 64 patients with insulinoma (40 women and 24 men; median age, 47.5 years; age range, 21 to 79) and 38 control subjects (25 women and 13 men; median age, 42.5 years; age range, 20 to 83) considered not to have a hypoglycemic disorder on the basis of normal results of a supervised 72-hour fast.

RESULTS

HbA(1) was significantly lower in patients with insulinoma (median, 4.7%; range, 2.7 to 6.9%) than in control subjects (median, 5.3%; range, 4.1 to 6.4%) (P<0.001, two-tailed rank sum test). Among 15 patients with insulinoma treated with diazoxide preoperatively, HbA(1) was higher (median, 4.8%; range, 4.2 to 6.9%) than in patients not treated with diazoxide (median, 4.6%; range, 2.7 to 5.7%), although the difference was not statistically significant (P = 0.08).

CONCLUSION

Because of considerable overlap in HbA(1) values, no GHb value was diagnostic for insulinoma; however, 16 of 64 patients (25%) with insulinoma had HbA(1) values below the lowest value (4.1%) in control subjects. Thus, HbA(1) values less than 4.1% in patients with possible insulinoma are strongly indicative of that disorder.

Nonenzymatically glycosylated proteins

Nonenzymatic glycosylation takes place in all proteins with a free-reacting lysine or valine in the presence of glucose. The formation of glycosylated plasma albumin, hemoglobin (Hb A1c), and skin collagen provides a diagnostic index of short- to long-term time-concentration of glucose in vivo. A wide range of assay methods are available, with affinity chromatographic, isoelectric focusing, and spectrophotometric methods providing the best accuracy and versatility. Glycosylated hemoglobin assays indicate glucose pressure over the previous 2 to 3 months and are of diagnostic value in general diabetic control, while glycosylated plasma albumin determinations are preferable in acute episodes in the life of a diabetic (e.g., pregnancy, infection, stress, trauma, surgery), since they provide an overview of changing blood glucose values of the previous 2 to 4 weeks. Glycosylated collagen estimations reflect tissue aging and are relevant in healing processes. Glycosylation alters the biologic activity of proteins, and these may relate to the manifold complications concomitant on the lifelong elevation of blood and tissue glucose in the diabetic (C6a). Assays for glycosylated hemoglobin have been routinely performed in clinical chemistry laboratories for a decade, and convenient determination for other nonenzymatically glycosylated proteins is proceeding apace.