An elderly patient presents with extreme hyperglycemia (blood glucose 780 mg/dL) and dehydration. Which diagnosis is most likely?

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Extreme hyperglycemia with dehydration in an elderly patient points to a hyperosmolar hyperglycemic state. In this condition, very high glucose levels cause massive osmotic diuresis and fluid loss, leading to profound dehydration. Because there is still some insulin activity, there is little to no ketone production, so there is minimal ketoacidosis. The resulting high serum osmolality drives neurologic symptoms, such as confusion or coma, which fits the presentation.

Diabetic ketoacidosis, by contrast, typically involves significant acidosis with ketones, and glucose elevations are often not as extremely high as seen here, plus it’s more common in younger patients with type 1 diabetes. A hypoglycemic coma happens when glucose is too low, not exceedingly high. Hyperglycemia due to steroids can raise glucose, but it usually does not reach the extreme levels with the marked dehydration and altered mental status characteristic of hyperosmolar hyperglycemic state.