Background <p>Acute hyperglycaemia is often accompanied by acid–base and electrolyte disorders as well as changes in serum osmolality which have a significant clinical impact. This study explores the prevalence of complex acid–base disorders in patients with hyperglycaemia, focusing on the limitations of current diagnostic criteria which primarily rely on pH, serum bicarbonate and anion gap.</p> Methods <p>A retrospective analysis of 1159 episodes of severe hyperglycaemia was performed. Arterial blood gas analysis, serum osmolality and electrolyte levels were measured at admission to hospital and patient’s outcome was observed until day seven. Patients were evaluated for acid–base and electrolyte disorders as well as for changes in measured or calculated serum osmolality.</p> Results <p>Our findings reveal that 90.7% of patients exhibited some form of acid–base disorder, mixed acid–base disorders were the most prevalent (75%). Patients with an accompanying respiratory acidosis showed higher mortality rates (12%) than patients with respiratory alkalosis (6%, <i>p</i> = 0.006) or no respiratory disorder (5%, <i>p</i> = 0.003). An elevated serum osmolality was associated with higher mortality when using the calculated, effective osmolality (19% vs 4%, <i>p</i> &lt; 0.001) as well as the measured osmolality (10% vs 4%, <i>p</i> = 0.001). Only 20% of our population had no electrolyte disorder. This group had a significantly lower mortality rate (2%) compared to patients with elevated or decreased sodium, potassium and phosphate levels.</p> Conclusion <p>Patients with severe hyperglycaemia often had complex acid–base and electrolyte disorders but current criteria for diagnosing diabetic ketoacidosis (DKA) and hyperosmolar hyperglycaemic state (HHS) ignore combined disorders of acid–base homeostasis, potentially influencing patient management and outcomes. </p>