To investigate the prevalence of folic acid deficiency in Queensland-wide data of routine laboratory measurements, especially in high-risk sub-populations.

Secondary health data analysis.

Analysis of routine folic acid tests conducted by Pathology Queensland (AUSLAB).

Female and male persons aged 0–117 years with routine folic acid testing between 1 January 2004 and 31 December 2015. If repeat tests on the same person were conducted, only the initial test was analysed (n 291 908).

Overall the prevalence of folic acid deficiency declined from 7·5 % before (2004–2008) to 1·1 % after mandatory folic acid fortification (2010–2015; P < 0·001) reflecting a relative reduction of 85 %. Levels of erythrocyte folate increased significantly from a median (interquartile range) of 820 (580–1180) nmol/l in 2008 before fortification to 1020 (780–1350) nmol/l in 2010 (P < 0·001) after fortification. The prevalence of folic acid deficiency in the Indigenous population (14 792 samples) declined by 93 % (17·4 v. 1·3 %; P < 0·001); and by 84 % in non-Indigenous residents (7·0 v. 1·1 %; P < 0·001). In a logistic regression model the observed decrease of folic acid deficiency between 2008 and 2010 was found independent of gender, age and ethnicity (ORcrude = 0·20; 95 % CI 0·18, 0·23; P < 0·001; ORadjusted = 0·21; 95 % CI 0·18, 0·23; P < 0·001).

While voluntary folic acid fortification, introduced in 1995, failed especially in high-risk subgroups, the 2009 mandatory folic acid fortification programme coincided with a substantial decrease of folic acid deficiency in the entire population.