Correction for insulin-dependent diabetes in maternal serum α-fetoprotein testing has outlived its usefulness

Objective: Historically, α-fetoprotein (AFP) levels in insulin-dependent diabetes (IDDM) have shown an approximately 20% decrement, and a correction factor is used to standardize multiples of the median (MOMs). With new laboratory methods and improved precision, we sought to re-evaluate the correctness of this approach. Study Design: Consecutive biochemical screens were conducted among 60,287 nondiabetic patients and 307 patients with IDDM. Analyses were conducted in one laboratory, and comparisons were made with use of standard formula weight adjustments including a 20% correction factor for IDDM. Patients were then stratified according to maternal weight. Results: Nondiabetic patients averaged 1.00 MOM, IDDM patients 0.91 MOM with no adjustments, 0.96 MOM adjusting for weight only, and 1.20 MOM adjusting for weight and diabetes status. To explain the “overcorrection,” analysis by maternal weight showed significant overrepresentation of IDDM patients at 175 pounds or above. In fact, the mean weight in pounds for nondiabetic subjects was 151 ± 35 and for those with IDDM 174 ± 52 (P< .001). With use of an upper limit weight cutoff of 200 pounds, results are within 4% of normal. Conclusions: With current methodologies, the 20% correction factor for IDDM erroneously overcorrects. Two hundred pounds is sufficent, the weight correction for diabetic status should be abandoned. (Am J Obstet Gynecol 2002;187:1084-6.)