Blood tests and kidneys
Effective osmolality
Effective (tonicity) and calculated serum osmolality, for DKA and HHS.
For education. Check against your unit's guidance and a second clinician before use.
mmol/L
Optional. Needed for the calculated (total) osmolality.
Fill in the boxes above to see the result.
Before you use this
- A very high osmolality with high glucose and little or no ketosis suggests hyperosmolar hyperglycaemic state (HHS), which needs a different fluid approach from DKA. HHS and mixed DKA/HHS are defined in the ISPAD and BSPED guidelines.
- A measured osmolality from the lab is more accurate. A big gap between measured and calculated values suggests another osmole (for example a toxic alcohol).
How this is calculated
- Effective osmolality = (2 × sodium) + glucose, all in mmol/L
- Calculated osmolality = (2 × sodium) + glucose + urea
- Urea moves freely into cells, so it doesn't pull water across cell membranes. Effective osmolality leaves it out.
Sources
- British Society for Paediatric Endocrinology and Diabetes (BSPED). Guideline for the management of children and young people under 18 years with diabetic ketoacidosis.
- Glaser N, et al. ISPAD Clinical Practice Consensus Guidelines 2022: diabetic ketoacidosis and hyperglycemic hyperosmolar state. Pediatr Diabetes 2022;23:835–56.
Checked by PICUlearn, 21 Sept 2026.