THE COGNITIVE OUTCOME OF VERY PRETERM INFANTS MAY BE POORER THAN OFTEN REPORTED - AN EMPIRICAL-INVESTIGATION OF HOW METHODOLOGICAL ISSUES MAKE A BIG DIFFERENCE

The effects of relying on outmoded IQ-test norms and the use of arbitr ary classifications of developmental delay on estimates of cognitive i mpairment of very preterm infants (VPI) was evaluated in a prospective population study. Cognitive assessments included the Griffiths test a t 5 and 20 months and the Columbia Mental Maturity Scales (CMM) and a vocabulary test (Aktiver Wortschatz Test, AWST) at 56 months of age. R ates of cognitive impairment of 321 very preterm infants (VPI; < 32 we eks gestation or < 1500g birth weight) were determined according to th e published test norms, to scores of a full-term control group (FC n = 321), and to scores from a representative sample of children (NC n = 431) of the same birth cohort. IQ-scores were higher in the FC and NC children than in the original standardisation sample (SS). Using the c oncurrent test norms (FC, NC) up to 2.4 times more VPI were identified as seriously impaired (<-2 SD) than if the published (outdated) norms were used. Serious developmental delay was underestimated when arbitr ary (e.g. DQ < 70) rather than across age comparable definitions (DQ < -2 SD) were used. VPI study drop-outs had mothers with lower education al qualifications and poorer cognitive developmental scores at 5 or 20 months of age. In conclusion, a lack of appropriate control groups an d use of arbitrary criteria for judging serious delay leads to large u nderestimations of cognitive impairment in VPI. Findings from previous uncontrolled studies of VPI need re-interpretation.