Objectives To investigate the association between the degree of patient dehydration on presentation with diabetic ketoacidosis (DKA) and clinical and laboratory parameters obtained on admission.

Design Prospective descriptive study.

Setting A tertiary care children's hospital.

Patients Thirty-nine paediatric patients (1 month–16 years) presenting with 42 episodes of DKA.

Intervention Clinical and biochemical variables were collected on admission. Dehydration was calculated by measuring acute changes in body weight during the period of illness. All patients were treated according to a previously established protocol.

Main outcome measures Magnitude of dehydration, defined as % loss of body weight (LBW), was determined by the difference in body weight obtained at presentation and at discharge. The relationship between the magnitude of dehydration and the clinical assessment and biochemical parameters was examined.

Results The median (25th–75th centiles) magnitude of dehydration at presentation was 5.7% (3.8–8.3%) (mean±SD 6.8±5%). Neither the initial clinical assessment nor the comprehensive biochemical profile at admission correlated with the magnitude of dehydration. Despite considerable variation in the degree of dehydration and biochemical disequilibrium, all patients recovered from DKA within 24 h with a standardised therapeutic approach. Furthermore, the rapidity of patient recovery did not correlate with the magnitude of dehydration on presentation or the amount of fluid administered (median (25th–75th centiles) 48.8 ml/kg (38.5–60.3)) in the first 12 h.

Conclusion The magnitude of dehydration in DKA is not reflected by either clinical or biochemical parameters. These findings need confirmation in larger studies.