The GDM of Diabetes

Dr. V. Seshiah,
Con. Diabetologist, Apollo Hospital, Chennai

Screening for abnormal glucose tolerance during pregnancy Abnormal glucose tolerance of any etiology, recognized or unrecognized, starting before pregnancy or revealed during pregnancy, is associated with a high risk of poor outcome of pregnancy. As glucose is toxic to the developing foetus, ideally all-pregnant women should undergo screening for glucose intolerance but screening is mandatory for high-risk patients likely to develop GDM (Table-1).

Table 1: Indications for Screening

Age > 25years
Family history of diabetes
Obesity (Pre-pregnancy BMI >25)
BOH - previous history of
Unexplained perinatal loss
IUD
Large for gestational age infant
Congenitally malformed infant
Polyhydroamnios
Pre-eclampsia
Glucose in second fasting

The expert committee on diagnosis and classification of diabetes has recommended that screening may not be necessary in women who fulfill the criteria given in table 2.

Table 2: 'Low-Risk' states where screening is not required

Age <25 years
Weight normal before pregnancy
Member of an ethnic group with a low prevalence of GDM
No known diabetes in first-degree relatives
No history of abnormal glucose tolerance
No history of poor obstetric outcome

Urine Glucose: Glucosuria is a commonly employed screening test for the detection of glucose intolerance. During pregnancy, the renal threshold for glucose is often lowered, due partly to an eight-fold increase in glomerular filtration of glucose, and partly, to an intermittent tubular defect in glucose re-absorption. Low renal threshold for glucose during pregnancy renders glucosuria less specific for detection of GDM and must not be used as a diagnostic test.

O'Sullivan's Screening Test: The screening test recommended by O'Sullivan and Mahan is to do blood glucose determination one hour after a 50 g oral glucose load. If plasma glucose is more than 140 mg/dl an oral GTT with 100 gms glucose is ordered.The short comings of this screening is that it is a 2 step procedure .Further, it requires 50 g of glucose challenge and the subject has to remain for an additional period of one hour in the clinic. As such this procedure is not suitable to follow in hospitals with high birth rates. Hence a simpler screening procedure "Spot Test" was worked out based on the study in Madras by Seshiah et.al (1). In their study it was observed that fasting plasma glucose or nonfasting plasma glucose never exceeded 90 mgs and 120 mgs respectively in normal pregnant women. Hence any pregnant woman whose plasma glucose value exceeds these cut-off points should be subjected to an oral glucose tolerance test with 75 gm glucose.

Table3: Current criteria for diagnosis of GDM using glucose load (Plasma Values)

100 g 75 g
mg/dl mmol/L mg/dl mmol/L
Fasting 95 5.3 95 5.3
1 hour 180 10.0 180 10.0
2 hour 155 8.6 155 8.6
3 hour 140 7.8

The latest diagnostic criteria recommended by the American Diabetic Association as the cut-off point for diagnosis of GDM is shown in table 3.Any 2 values above the level recommended is considered to be GDM. (2)

WHO Criteria: A standard OGTT should be performed after over night fasting by giving 75gm of glucose. Plasma glucose is measured at fasting and after 2 hours. Pregnant women who meet WHO criteria for IGT and diabetes are classified as having gestational diabetes mellitus (GDM). (Table 4) (3).

Table 4: WHO Criteria (Plasma Glucose)

FPG (mg/dl) 2h PG (mg/dl)
IGT< 126 140-200
Diabetes >126 >200

As per WHO criteria, FPG <126 mg/dl is normal, whereas by ADA criteria FPG <95 mg/dl is normal. The 2 hour post plasma glucose of >140 mg/dl by WHO criteria and >155 mg/dl by ADA criteria are abnormal. With the effective treatment available, the WHO criteria of 2hour PPG > 140mg/dl and ADA criteria of FPG>95 mgs (2,4) by identifying a large number of cases have greater potential for prevention of fetal morbidity. Hence, we can adopt for our screening procedure FPG >95 and 2 hour PPG > 140 as abnormal.

GTT With 75gm Glucose

ADA WHO
FPG >95 <126
2 HOUR PPG >155 >140

If any one of the criteria are fulfilled i.e., fasting or 2 hr more than the recommended values should be considered as having IABG (Isolated abnormal blood glucose) and followed similar to GDM. If the screene has fasting plasma glucose more than 126 mgs and 2 hr Post glucose more than 200 mgs, probably she has been having undetected diabetes prior to conception (pregestational diabetes) which can be confirmed by glycosylated haemoglobin.

The above suggested criteria is only for the diagnosis of GDM. For treatment of glucose intolerance during pregnancy the effort should be to maintain fasting 90+/-5 mgs and 2 hr post meal of 120+/-5 mgs (2) so that a mean blood glucose around 105 mgs/ dl is maintained as this assures best fetal outcome.