Parretti et al. (1) were very meticulous in assuring that their subjects had similar demographics and were glucose tolerant, healthy, and of similar gestational age. Of note is their finding of a mean fasting glucose value of 56.2 mg/dl and mean 24-h glucose values of 74.7 mg/dl. These results are substantially lower than those found by previous investigators who reported fasting plasma glucose concentrations between 78 and 80 mg/dl (24) and 24-h mean glucose concentrations between 85 and 96 mg/dl (2,5,6). Before one accepts the results of the study by Parretti et al. as representative of those of nondiabetic women in the third trimester, it seems appropriate to question whether differences in methodology might explain these seeming inconsistencies.

The meter (Accutrend α) that was used in their study is calibrated to give results equivalent to whole blood, not plasma glucose. Because the concentration of glucose in plasma is greater than that of red cells, the higher the hematocrit, the higher the plasma glucose corresponding to the same patient’s whole-blood glucose concentration (7). For example, assuming a hematocrit of 35%, the plasma glucose equivalent of the fasting and overall daily mean glucose reported in the study were 65.4 and 87.0 mg/dl, respectively. Such a conversion brings the mean glucose concentrations reported by the authors into closer proximity with those reported by others.

In contrast with previous studies, the authors had patients check and report their own glucose level. The protocol required that each subject report a total of 15 blood glucose tests taken around the clock for 1 day every 2 weeks from 28 to 38 weeks. The Accutrend α has a memory capacity for only nine results and cannot be downloaded to a computer. Thus, it seems likely that the patients’ results were self reported and not transcribed directly from the meter. Unless the investigators verified these results, either by direct observation or by running duplicate saved samples independently in the laboratory, the credibility of these results may legitimately be called into question.

Hopefully these comments will aid in the interpretation and application of the findings of this unique study.

Parretti E, Mecacci F, Papini M, Cioni R, Carignani L, Mignosa M, La Torre P, Mello G: Third-trimester maternal glucose levels from diurnal profiles in nondiabetic pregnancies: correlation with sonographic parameters of fetal growth.
Diabetes Care
Cousins L, Rigg L, Hollingsworth D, Brink C, Aurand J, Yen SSC: The 24-hour excursion and diurnal rhythm of glucose, insulin, and C-peptide in normal pregnancy.
Am J Obstet Gynecol
Meis PJ, Rose JC, Swain M: Pregnancy alters diurnal variation of plasma glucose concentration.
Chronobiol Int
Weiss PAM, Haeusler M, Kainer F, Purstner P, Haas J: Toward universal criteria for gestational diabetes: relationships between seventy-five and one hundred gram glucose loads and between capillary and venous glucose concentrations.
Am J Obstet Gynecol
Gillmer MDG, Beard RW, Brooke FM, Oakley NW: Carbohydrate metabolism in pregnancy. I. Diurnal glucose profile in normal and diabetic women.
Br Med J
Phelps RL, Metzger BE, Freinkel N: Carbohydrate metabolism in pregnancy. XVII. Diurnal profiles of plasma glucose, insulin, free fatty acids, triglycerides, cholesterol, and individual amino acids in late normal pregnancy.
Am J Obstet Gynecol
Dillon RS: Importance of the hematocrit in interpretation of blood sugar.

Address correspondence to David A. Sacks, MD, Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Kaiser Foundation Hospital, Bellflower, CA 90706. E-mail: