Gestational Diabetes Mellitus: Screening and Diagnosis

Transcription

1 SYMPOSIUM Gestational Diabetes Mellitus: Screening and Diagnosis Gestational diabetes mellitus (GDM) represents a major health problem, affecting 2% to 3% of the obstetric population, or greater than 100,000 women annually in the United States. Recent data suggest a program of universal screening for GDM is a reasonable approach, as reliance upon historical and clinical risk factors as well as age cutoffs for screening does not appear sensitive enough. Screening should be accomplished by measuring blood glucose levels after a 1 -hour time period after ingestion of a SO-g glucose load. A 3-hour diagnostic oral glucose tolerance test is performed if a screening threshold of 135 to 140 mg/dl (7.49 to 7.77 mmol/l) (plasma) is exceeded. The National Diabetes Data Group modification of the criteria of O'Sullivan and coworkers for the diagnosis of GDM should be employed until we can better define the level of glycemia that poses a risk for fetal and neonatal complications such as macrosomia. From the Department of Obstetrics and Gynecology, The Ohio State University, College of Medicine, Columbus, Ohio. Reprint requests to Department of Obstetrics and Gynecology, The Ohio State University, College of Medicine, 1654 Upham Dr, Means Hall, 5th Floor, Columbus, OH (Dr Landon). D iabetes mellitus represents the most common medical complication of pregnancy, with 2% to 3% of pregnant women being affected. Approximately 90% of these women will have gestational diabetes mellitus (GDM), a state restricted to women whose impaired glucose tolerance is first discovered during pregnancy.1 GDM is a major health problem, as over 100,000 cases are diagnosed annually in the United States. It is important to identify the pregnant woman with GDM because this disorder is associated with significant risks for perinatal and maternal morbidity.2 An increased incidence of macrosomia, birth trauma, hypoglycemia, hyperbilirubinemia, hypocalcemia, and polycythemia are consistently reported in the offspring of women with GDM.2'3 A history of stillbirth in prior pregnancies is commonly reported in women with GDM, suggesting that undetected and untreated GDM may result in fetal death. Finally, patients with GDM represent a group with significant risk for developing glucose intolerance later in life; it has been reported that 60% of these patients will become diabetic in the 16 years following pregnancy.4 Patients with marked fasting hyperglycemia (>130 mg/dl [7.22 mmol/l]) during pregnancy are at greatest risk for subsequent diabetes, with obesity being another strong predictor.5 D iagnosis of GDM The 100-g oral glucose tolerance test (OGTT) has become the accepted method for the diagnosis of GDM in the United States.1 A 75-g OGTT that employs more stringent criteria to document diabetes mellitus (WHO criteria) remains popular in Europe and other countries. As the WHO criteria for the diagnosis of GDM using the 75-g glucose load do not differ from the nonpregnant state, this test will identify fewer cases of carbohydrate intolerance during gestation. Compared with the intravenous test, the OGTT is far simpler to administer to a large outpatient population and more physiologic as it assesses the gastrointestinal factors involved in insulin secretion. The criteria recognized for the diagnosis of GDM using the 100-g OGTT were first proposed by O'Sullivan and Mahan.6 Because plasma is now more widely assayed than whole blood, the National Diabetes Data Group modified these threshold values by multiplying each value by 1.14 and then rounding off the value to the nearest 5 mg/dl (Table). The OGTT criteria by O'Sullivan and Mahan are based on established means and distributions of fasting, 1-, 2-, and 3-hour blood glucose values in an unselected sample of 752 pregnant women in the second and third trimester. These authors carefully recorded Mark B. Landon, MD Uboratory Medicine Vol. 21, No. 8 August

2 Oral Glucose Tolerance Test Values for Pregnancy* Time, h Whole Blood, mg/dl (mmol/l) Plasma, mg/dl (mmol/l) (100-gload) Fasting (0) (5.00) 165(9.16) 145 (8.05) 125(6.94) 105 (5.83) 190(10.55) 165 (9.16) 145 (8.05) the presence of any family history of diabetes, perinatal outcome, and any later development of diabetes. The subsequent onset of diabetes was significantly increased when 3-hour OGTT values exceeded the mean by 2 SDs (16% compared with less than 1 % in women with normal OGTT values at 5-year follow-up). Diabetes developed in 40% of women with the test values exceeding the mean by 3 SDs. Further studies documented an increased risk of perinatal mortality in women with OGTT values exceeding the mean by 2 SDs, particularly in women older than 25.7 For all of these classifications, O'Sullivan and Mahan required that two or more test levels be met or exceeded to avoid misclassification from a single high value secondary to rapid absorption of glucose. S creening for GDM An ideal screening test should have high sensitivity and specificity. In the case of GDM, the benefits of effectively identifying a group of pregnant women at risk for significant perinatal morbidity far outweigh the patient anxiety generated from a false-positive screening test result. In contrast, a false-negative diagnosis may delay or prevent therapy, resulting in perinatal complications such as macrosomia and even fetal death. For these reasons, screening methodologies for GDM have been designed to have high sensitivity and a moderately high specificity, with an abnormal screening value being present in approximately 15% to 20% of the population studied. The screening test for GDM by O'Sullivan and colleagues,8 the 50-g Laboratory Medicine Vol. 21, No. 8 August 1990 women who exhibit normal screening test results during the first 20 weeks of pregnancy.10 Similarly, women with positive screening test results during mid-pregnancy and normal OGTT values, if retested in the mid-third trimester may then demonstrate glucose intolerance consistent with GDM.9 The clinical significance of this late diagnosis remains uncertain, yet it does raise the question of the reproducibility of the OGTT values during pregnancy as well as the effect of gestational age upon testing.11 Some investigators, therefore, have suggested establishing reference values for each trimester for glucose tolerance testing.12 The Second International Workshop-Conference on GDM suggested that the 1-hour 50-g oral glucose screening test could be given without regard to time of the last meal or time of day.1 This approach is simple and convenient for the patient in that scheduling and dietary preparation are unnecessary. However, Coustan and colleagues13 demonstrated that the threshold for performing a diagnostic OGTT may need to be lowered to 130 mg/dl (7.22 mmol/l) (plasma) if the test is administered in the fed state. In their study, when the test was administered 1 hour after a meal, a 140 mg/dl (7.77 mmol/l) threshold detected only 7 1 % of women with GDM. These authors hypothesize that the lower screening value observed in the fed state occurs because the previous ingestion of a mixed meal dampens the usual state of insulin resistance observed in fasting pregnant women.13 While the 1-hour time period is clearly more efficient for screening a large number of women than a 2-hour window, it has been suggested that a plasma glucose test administered 2 hours after a 50-g plasma load may considerably improve specificity." Wiener and colleagues14 compared a 1hour cutoff value of greater than 139 mg/dl (7.71 mmol/l) with a 2-hour value of greater than 117 mg/dl (6.49 mmol/l) in 185 women and demonstrated that a 34% reduction in the number of follow-up OGTTs would be required using a 2-hour specimen as an index. Because only 4 women with GDM were identified in this study and the difficulty most patients *The diagnosis of gestational diabetes is made if two or more values meet or exceed the thresholds above. hour test, has become the "gold standard" screening test. Clinical features such as prior birth of a macrosomic infant, family history of diabetes, and glycosuria have been found to be ineffective as a screening method when compared with the 50-g oral glucose challenge.8 O'Sullivan and colleagues administered the 50-g screening test followed by a subsequent 3-hour OGTT to the population of 752 women described above. A cutoff value of 130 mg/dl (7.22 mmol/l) in whole blood (140 mg/dl [7.77 mmol/l] in plasma) was selected for analysis. A total of 109 women (14.5%) had abnormal screening results, and 13.7% of these women were subsequently found to have an abnormal 3-hour OGTT result. Of significance, only 4 of the 643 women had an abnormal 3hour test result, when the 50-g screen was less than 130 mg/dl (7.22 mmol/ L). The 50-g screen had a sensitivity of 79% (15 of 19 cases) and a specificity of 87%, leading O'Sullivan and coworkers to conclude that this screening method was superior to clinical risk factors alone for the identification of women with GDM. While the description of the 50-g glucose challenge by O'Sullivan and colleagues did not specify the gestational age appropriate for testing, most practitioners now initiate testing between 24 and 28 weeks of pregnancy. Few false-negative results are obtained during this period as the diabetogenic stress of pregnancy is most evident during the third trimester. Securing the diagnosis of GDM allows for 10 to 14 weeks of therapy that consists of diet and/or insulin administration. Since maternal glucose homeostasis is impaired with advancing gestation, there seems to be little benefit to universal screening in the first trimester for GDM.' Jovanovic and Peterson9 were able to detect only one case of GDM in 300 women who were screened between 9 and 20 weeks of gestation. Therefore, many practitioners perform early screening only in women with a history of GDM, morbid obesity, or significant glycosuria. Women with elevated glucose screening test results and normal 3-hour OGTT values early in pregnancy are, however, at increased risk for development of GDM when compared with

3 A lternative Methods for Screening During the past decade, blood glucose self-monitoring with the use of portable reflectance meters has become an important adjunct in the care of pregnant women with insulin-dependent diabetes. The accuracy of reflectance meters for the assay of capillary glucose specimens has been well documented. Several investigators have determined the utility of outpatient screening for GDM with the use of a reflectance meter.1617 Landon and colleagues determined that the sensitivity and specificity with a meter cutoff value of 160 mg/dl (8.88 mmol/l) were 93% and 96%, respectively, when compared with a venous plasma threshold of 135 mg/dl (7.49 mmol/l).17 The capillary-venous difference observed most likely represents the true higher glucose concentration of fingerstick blood rather than methodologic differences in the assay techniques. The fingerstick method has advantages over venous plasma sampling; patients with abnormal screening values are promptly identified and scheduled for a 3-hour OGTT. Delays in diagnosis and treatment are thus avoided with consider- able cost savings. However, reflectance meter determinations performed in an office setting require adherence to guidelines for calibration, maintenance, and quality control to assure accuracy. For these reasons, some have advocated using only central laboratory assays for screening for GDM.18 Further bias may also be introduced with the fingerstick method if one uses cutoff values reported in the literature without establishing a proper threshold for screening a particular population. Measurements of glycosylated serum proteins including hemoglobin have been correlated with glycemic control in nonpregnant individuals, leading investigators to determine whether such tests may be effective in screening for GDM. Glycosylated hemoglobin measurements have been most widely studied and do not seem to be helpful in the detection of GDM.19'20 Cousins and colleagues" screened 806 prenatal patients and compared the 1-hour 50g test with total hemoglobin A1 concentration using an ion-exchange technique. Among healthy subjects, glycosylated hemoglobin levels ranged from 4.0% to 10.9%, whereas among carbohydrate-intolerant subjects, levels ranged from 6.9% to 11.5%. The considerable overlap in these groups results in poor specificity for this method. Approximately, 40% of the population screened with glycosylated hemoglobin would require OGTT testing in order to achieve comparable sensitivity (80%) to the standard 1hour 50-g screen in which only 9.1% of women required the 3-hour OGTTs. Morris et al21 are among the few who support the use of glycosylated hemoglobin for GDM screening. These investigators measured glycosylated hemoglobin using affinity chromatography in 64 women at 10 to 15 weeks of gestation. Gestational diabetes developed in 15 women who demonstrated a higher glycosylated hemoglobin level when compared with healthy pregnant women. Using a cut-off of 6.3%, the authors noted a sensitivity of 93%. This study assessed the utility of this method in a selected population in early pregnancy and did not compare glycosylated hemoglobin as an indicator of diabetes in pregnancy with the standard 50-g screen performed in later gestation. While the newer affinity chromatography technique utilized by Morris et al is preferable to ion-exchange techniques for determining glycosylated hemoglobin concentrations, other investigators using this newer technique have reported it to be an insensitive screening tool for GDM.22 The failure of glycosylated hemoglobin to adequately select women with GDM may in part be explained by the inability of such measurements to identify subtle degrees of postprandial hyperglycemia, which may not be evident prior to the third trimester. Glycosylated albumin and glycosylated protein (fructosamine) have a significantly shorter half-life than glycosylated hemoglobin and have been proven to be useful indices of recent changes in blood glucose levels. Roberts and Baker23 demonstrated that 85% of women with gestational diabetes have higher serum levels of fructosamine than nondiabetic pregnant women. A limited follow-up study from this group involving 63 women (7 cases of GDM) tested within 4 weeks of the diagnostic OGTT resulted in a sensitivity, specificity, positive predictive value, and negative predictive value of 86%, 95%, 66%, and 98%, respectively. In contrast, Ryan and associates22 demonstrated considerable overlap in both glycosylated albumin and glycosylated protein values in 20 nondiabetic pregnant women compared with 20 women with the recent diagnosis of GDM. These authors concluded that glycosylated albumin and glycosylated protein are of little value in the diagnosis of GDM. They reported sensitivities of only 8% and 3% for glycosylated albumin and glycosylated protein, respectively. U niversal vs Selective Screening for GDM Most practitioners screen all pregnant women for GDM.24 A total of 90% of maternal-fetal specialists and approximately 77% of general obstetrician-gynecologists screen every patient in their practice.24 The highly prevalent practice of universal screening is supported by the Second International Workshop-Conference on Gestational Diabetes Mellitus.1 However, recommendations for universal have waiting 2 hours for phlebotomy, it seems unlikely that this method will gain popularity. Random blood glucose testing would seem to be desirable for screening as it does not inconvenience patients and might also be more cost efficient. Lind15 determined a 99% cutoff value of 110 mg/dl (6.11 mmol/l) within 2 hours of a meal compared with 101 mg/dl (5.61 mmol/l) more than 2 hours postprandially in 2,403 pregnant women between 28 and 32 weeks of gestation. Because this study population did not routinely undergo glucose tolerance testing, appropriate thresholds are lacking, making it impossible to determine the clinical utility of this approach. In practice, if a fasting glucose level exceeds 99 mg/dl (5.49 mmol/l) or the 2-hour postprandial glucose level exceeds 129 mg/dl (7.16 mmol/l), it is advisable to perform a fasting glucose test. If this value is elevated, the diagnosis of GDM can be made. If not, a 3-hour OGTT should be scheduled. Laboratory Medicine Vol. 21, No. 8 August

4 ly. The largest prospective study concerning screening for GDM has recently been reported by Coustan et al.30 In this study of 6,214 women with 125 cases of GDM, the sensitivity using a 30-year age cutoff was only 65%. This figure reached 85% if the age cutoff was decreased to 25 years, while the cost per case diagnosed increased only 1%. The cost per case diagnosed with universal screening was $222 compared with $192 if the 25year age cutoff was employed. This modest cost savings must be balanced against missing as much as 22% of cases of GDM, when all women older than 24 are screened. It is difficult to assign a dollar figure to the preventable morbidity and mortality associated with undetected cases of GDM, another argument in favor of universal screening. Using historical risk factors in selecting patients for screening assumes that a careful review of medical records be incorporated into a decision made to perform glucose screening.30 Universal screening is a simplified approach in which all women are scheduled for the glucose challenge test at 24 to 28 weeks of gestation, thereby eliminating much of the potential for error in a busy clinic or practice. N ew Criteria for Diagnosing GDM As the criteria for the diagnosis of GDM by O'Sullivan et al are primarily related to the subsequent development of overt diabetes mellitus, several authors have challenged the validity of these criteria as they relate to perinatal outcome.31'32 Tallarigo and coworkers31 suggested that subtle degrees of maternal hyperglycemia, levels below those which would classify an individual as a gestational diabetic, can have a determined effect on perinatal outcome. In their study, a positive correlation between birth weight and the 2-hour OGTT value was observed. The incidence of macrosomia increased progressively as the 2-hour value increased, such that women with 2-hour values of 120 to 160 mg/dl (6.66 to 8.88 mmol/l) had a 27.5% rate of macrosomia. Herman and Raimonds32 similarly reported a 24% frequency of macrosomia in women with a 2-hour value exceeding 139 mg/dl (7.71 mmol/l). On the basis of the data from these studies, between 33% and 44% of the obstetric population would fall into a range of high normal glucose tolerance.32 Women with abnormal 50-g screening tests and normal 3-hour OGTTs (15% to 20% of the obstetric population) may also have an increased risk for delivering a macrosomic infant.33 Although recent preliminary data suggest a benefit to treating women with one abnormal 3hour OGTT value with dietary modifications, large prospective studies should be conducted to determine the clinical usefulness of such strategies before the criteria for establishing the diagnosis of GDM are significantly altered 3 ^ 530 Laboratory Medicine Vol. 21, No. 8 August 1990 References 1. Freinkel N, Gabbe SG, Hadden DR, et al: Summary and recommendations of the Second International Workshop-Conference on Gestational Diabetes Mellitus. Diabetes 1985;34: Oh W: Neonatal Outcome and Care in Diabetes Mellitus in Pregnancy, Reece EA, Coustan DR (eds). New York, NY, Churchill Livingston, Landon MB, Gabbe SG: Antepartum fetal surveillance in gestational diabetes mellitus. Diabetes 1985;34(suppl 2): O'Sullivan JB: Body weight and subsequent diabetes mellitus. JAMA 1982;248: Metzger BE, Bybee DE, Freinkel N, et al: Gestational diabetes correlation between the phenotypic and genotypic characteristics of the mother and abnormal glucose tolerance during the first year postpartum. Diabetes 1985;34(suppl 2): O'Sullivan JB, Mahan CM: Criteria for the oral glucose tolerance test in pregnancy. Diabetes 1964;13: O'Sullivan JB, Charles D, Mahan C, et al: Gestational diabetes and perinatal mortality rate. Am J Obstet Gynecol 1973;116: O'Sullivan JB, Mahan CM, Charles D, et al: Screening criteria for high risk gestational diabetic patients. Am J Obstet Gynecol 1973;116: Jovanovic L, Peterson CM: Screening for gestational diabetes: Optimum timing and criteria for retesting. Diabetes 1985;34(suppl 2): Sacks DA, Abu-Fadil S, Karter GJ, et al: Screening for gestational diabetes with the onehour 50 gm glucose test. Obstet Gynecol 1987; 70: Sacks DA, Abu-Fadil S, Greenspoon JS, et al: How reliable is the 50-gram, one-hour glucose screening test? Am J Obstet Gynecol 1989;161: Forest JC, Garrido-Russo M, Lemay A, et al: Reference values for the oral glucose tolerance test at each trimester of pregnancy. Am J Clin Pathol 1983;80: Coustan DR, Widness JA, Carpenter MW, et al: Should the fifty-gram one-hour plasma glucose screening test for gestational diabetes be administered in the fasting or fed state? Am J Obstet Gynecol 1986;154: Werner CP, Fraser MM, Burns JM, et al: Cost efficiency of routine screening for diabetes in pregnancy: 1-hour versus 2-hour specimen. Diabetes Care 1986;9: Lind T: Antenatal screening using random blood glucose values. Diabetes 1985;34(suppl 2): Landon MB, Cembrowski GS, Gabbe SG: Capillary blood glucose screening for gestational diabetes: A preliminary investigation. Am J Obstet Gynecol 1986;155: Weiner CP, Fanstich MW, Burns J, et al: Diagnosis of gestational diabetes by capillary blood samples and a portable reflectance meter: Derivation of threshold values and prospective validation. Am J Obstet Gynecol 1987;156: Carr S, Coustan DR, Martelly P, et al: Precision of reflectance meters in screening for gestational diabetes. Obstet Gynecol 1989;73: screening are in contrast to the 1986 American College of Obstetricians and Gynecologists Technical Bulletin, which suggested that screening be performed only in women 30 years of age or older unless specific risk factors are present.25 Over 15 years ago, O'Sullivan et al and others demonstrated that screening only those women with historical or clinical risk factors had a yield of approximately 50% in detecting cases of GDM.8'26 In a study of 2,027 women, Lavin27 confirmed the inadequacy of selective screening; only 46% of cases of GDM were found in patients with risk factors.27 Some authors have advocated screening only those women over age Marquette et al,28 using a plasma threshold of 150 mg/dl and an age cutoff of 25 years, reported a sensitivity of 88%, with a cost per case diagnosed that approached 50% less than universal screening in their population. Singer and colleagues29 calculated similar savings by employing the 25-year age criteria for screening. In their analysis of 10,000 women, the total cost for universal screening was $96,240 compared with $49,896 using the 25-year age cutoff. Universal screening detected 83% of cases (O'Sullivan's method) compared with 71 % of cases using the 25-year age criteria, with the cost per case diagnosed as $387 and $325, respective-

5 19. Cousins L, Dattel BJ, Hollingworth DR, et at: Glycosylated hemoglobin as a screening test for carbohydrate intolerance in pregnancy. Am J Obstet Gynecol 1984;150: Shah BD, Cohen AW, May C, et al: Comparison of glycohemoglobin determination and the one-hour glucose screen in diabetes. Am J Obstet Gynecol 1982;144: Morris MA, Grandis AS, Litton J: Glycosylated hemoglobin: A sensitive indicator of gestational diabetes. Obstet Gynecol 1986;68: Ryan EA, Stark R, Crockford PM, et al: Assessment of value of glycosylated albumin and protein in detection of gestational diabetes. Diabetes Care 1987;10: Roberts AB, Baker JR: Serum fructosamine: A screening test for diabetes in pregnancy. Am J Obstet Gynecol 1986;154: Landon MB, Gabbe SG, Sachs L: Management of diabetes mellitus and pregnancy: A survey of obstetricians and maternal-fetal specialists. Obstet Gynecol 1990;75: American College of Obstetricians and Gynecologists: Management of diabetes mellitus in pregnancy. ACOG technical bulletin no. 92, Washington, DC, American College of Obstetricians and Gynecologists, Beard RW, Gilmer MDG, Oakley NW, et al: Screening for gestational diabetes. Diabetes Care 1980;3: Lavin JP: Screening of high risk and general populations for gestational diabetes. Diabetes 1985;34(suppl2): Marquette GP, Klein VR, Repke JT, et al: Cost-effective criteria for glucose screening. Obstet Gynecol 1985;66: Singer DE, Samet JH, Coley C, et al: Screening for diabetes mellitus. Ann Intern Med 1988; 109: Coustan DR, Nelson C, Carpenter MW, et al: Maternal age and screening for gestational diabetes: A population-based study. Obstet Gynecol 1989;73: Tallarigo L, Giampietro O, Genno, et al: Relation of glucose tolerance to complications of pregnancy in nondiabetic women. N Engl J Med 1986;315: Herman G, Raimonds B: Glucose tolerance, fetal growth, and pregnancy complications in normal women. Am J Perinatal 1988;5: Leikin EL, Jenkins JM, Pomerantz GA, et al: Abnormal glucose screening tests in pregnancy: A risk factor for fetal macrosomia. Obstet Gynecol 1987;69: Langer O, Brustman L, Anyaegbunam A, et al: The significance of one abnormal glucose tolerance test value on adverse outcome in pregnancy. Am J Obstet Gynecol 1987;157: Uboiatory Medicine Vol. 21, No. 8 August

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