DKA vs HHS: How to Tell Them Apart
DKA and HHS are the two life-threatening hyperglycemic crises of diabetes, and both require fluids, insulin, potassium replacement, and identification of a precipitant. The core axis that separates them is the degree of insulin deficiency: DKA reflects absolute insulin deficiency producing ketosis and anion-gap acidosis, whereas HHS reflects relative insulin deficiency with enough insulin to suppress ketogenesis but not to control glucose, resulting in extreme hyperglycemia and hyperosmolality without significant ketosis.
How to tell them apart
| Feature | Diabetic ketoacidosis (DKA) | Hyperosmolar hyperglycemic state (HHS) |
|---|---|---|
| Ketosis | Significant ketosis; ketones strongly positive | No significant ketosis — enough insulin remains to prevent lipolysis and ketogenesis |
| Acid-base status | Anion-gap metabolic acidosis with low pH and low bicarbonate | No significant acidosis; the problem is hyperglycemia and hyperosmolality |
| Insulin deficiency | Absolute insulin deficiency (classically Type 1 diabetes running out of insulin) | Relative, not absolute, insulin deficiency |
| Blood glucose | Usually markedly elevated, but can be near-normal in euglycemic DKA (e.g., with SGLT2 inhibitors) | Extremely high glucose driving profound hyperosmolality |
| Onset | Develops relatively acutely | Develops insidiously over days to weeks |
| Typical patient | Often younger patients with Type 1 diabetes; can be precipitated by illness, fasting, or SGLT2 inhibitor use | Often elderly patients with limited access to water or impaired thirst |
| Dehydration and osmolality | Dehydration present but less extreme | Profound dehydration (may be 8–10 L down) with extreme hyperosmolality causing altered mental status and coma |
| Mortality / danger | Dangerous but lower mortality than HHS | More dangerous with higher mortality due to profound dehydration and hyperosmolality — fluids are the priority |
The reasoning
Anchor on ketosis and acidosis. If the patient has positive ketones plus an anion-gap acidosis, you are dealing with DKA regardless of the glucose level. If instead there is extreme hyperglycemia with profound dehydration, hyperosmolality, and altered mental status but no significant ketosis or acidosis, the diagnosis is HHS. Remember the pathophysiologic hinge: HHS patients retain just enough insulin to block ketogenesis but not hyperglycemia, so they escape the acidosis. In DKA, do not stop the insulin drip when glucose normalizes — continue until the anion gap closes. In HHS, fluid resuscitation is the top priority because these patients are the most volume-depleted.
Key tests
- Serum/urine ketones: strongly positive in DKA; minimal in HHS
- Arterial pH, bicarbonate, and anion gap: low pH, low bicarbonate, and an elevated anion gap in DKA; no significant acidosis in HHS
- Blood glucose: markedly elevated in both, but can be near-normal in euglycemic DKA and is extremely high in HHS
- Serum osmolality: extreme hyperosmolality in HHS, correlating with altered mental status and coma
What they share
- Both are acute hyperglycemic crises of diabetes
- Both are managed with IV fluids, insulin, potassium replacement, and identification/treatment of the precipitant
- Both require close potassium monitoring, since insulin shifts potassium into cells and can precipitate dangerous hypokalemia
- Both can be triggered by an intercurrent illness or precipitating event
Pitfalls
- Being fooled by a near-normal glucose in euglycemic DKA (classically with SGLT2 inhibitors) — the acidosis and ketosis still define DKA and require the same treatment
- Stopping the insulin drip in DKA once glucose normalizes; you must continue insulin until the anion gap closes
- Misreading the initially high serum potassium in DKA as adequate stores — total body potassium is depleted, and insulin will drop serum potassium precipitously, risking fatal arrhythmias
- Giving bicarbonate in DKA when it is rarely indicated — reserve it for very low pH (below 6.9)
- Attributing abdominal pain in DKA to a primary surgical abdomen; abdominal pain is common in DKA and improves with treatment of the DKA itself
- Underestimating HHS: its insidious onset and profound dehydration make it more lethal than DKA
Practice this the way the exam tests it — on branching cases where your decisions shape the patient.