To the Editor: The 50 g glucose challenge test (GCT) is widely recommended as a screening test for gestational diabetes (GD).1 The test consists of a 50 g oral glucose load given at any time of the day, followed one hour later by the measurement of the plasma glucose concentration.2 This test is recognised as imperfect for screening, as sensitivity and specificity are not 100%.2,3 It is known that glucose tolerance deteriorates in the afternoon,4 which raises the question of whether time of day influences the response to the 50 g GCT.

At Royal North Shore Hospital, screening for GD is performed at the 26–28-week visit by means of the 50 g GCT. In 2000, screening for GD was introduced into a morning midwives antenatal clinic, whereas previously it had only been performed in the afternoon. The population attending the clinic at the 26–28-week visit includes many women receiving shared care, and is regarded as being at low obstetric risk.

The Table shows the results of screening at the morning clinic compared with screening in the afternoon over the same time period. The two groups were identical in terms of age, weight, ethnicity, and family history of diabetes or past history of GD. The percentage of women with a positive screening test result during the morning clinic (17.0%) was significantly lower than that during the afternoon clinic (31.1%). Positive screening results were followed up with a diagnostic 75 g glucose tolerance test, and GD was diagnosed according to the Australian Diabetes in Pregnancy Society criteria.5 Women with a positive screening test result confirmed with a 75 g glucose tolerance test in the afternoon were less likely to have GD than those with a positive test in the morning (31.5% v 40.0%). Despite the fact that a smaller percentage of women who screened positive in the afternoon had GD, a greater percentage of the total number screened in the afternoon had GD than in the morning group. In this cohort, the difference (9.8% v 6.8%) was not significant (Table; P = 0.15).

These results are consistent with the hypothesis that a 50 g GCT test performed in the afternoon results in a greater number of positive results, a greater number of women undergoing diagnostic testing and a greater number of women identified with GD. The morning GCT appears to increase specificity, with an associated decrease in sensitivity.

These results need to be taken into consideration when designing or implementing a screening program.