Hyperosmolar hyperglycemic state (HHS) does not typically cause significant metabolic acidosis on its own. By definition, HHS is diagnosed when arterial pH remains above 7.3 and serum bicarbonate stays above 18 mEq/L, both markers of relatively preserved acid-base balance. However, about half of HHS patients do develop a mild degree of metabolic acidosis, and roughly one in four presents with a true overlap of HHS and diabetic ketoacidosis (DKA), which absolutely does cause acidosis. So the short answer is nuanced: pure HHS avoids the severe acidosis seen in DKA, but mild or secondary acidosis is common.
Why HHS Typically Avoids Acidosis
The key difference between HHS and DKA comes down to insulin. In HHS, the body still produces enough insulin to block the breakdown of fat into ketone bodies, the acids responsible for the dangerously low pH in DKA. The amount of insulin needed to prevent fat breakdown is surprisingly small, only about one-tenth of what’s needed to help cells absorb glucose. That’s why someone with HHS can have blood sugar above 600 mg/dL and still not develop significant ketoacidosis. Their insulin supply is inadequate for glucose control but sufficient to keep fat metabolism in check.
Several other factors also help suppress ketone production in HHS. Patients tend to have lower levels of stress hormones that promote fat breakdown, and the extreme hyperosmolarity of the blood itself appears to inhibit the release of fatty acids from fat tissue. Of these mechanisms, the residual insulin secretion is considered the most important.
When Mild Acidosis Does Occur
Despite the textbook distinction, pure HHS without any metabolic acidosis is less common than many people expect. According to the American Academy of Family Physicians, about half of HHS patients have a mild anion gap metabolic acidosis, with an anion gap in the range of 10 to 12. This mild acidosis doesn’t come from ketone bodies. Instead, it typically results from other consequences of the crisis itself.
The most common secondary cause is lactic acidosis. HHS causes severe dehydration, sometimes with fluid deficits of 8 to 10 liters. That level of volume loss reduces blood flow to tissues, which forces cells to generate energy without adequate oxygen. The byproduct is lactic acid. Poor kidney function from dehydration also makes it harder for the body to clear acids normally, compounding the problem. So while HHS doesn’t generate ketoacids the way DKA does, the dehydration and circulatory stress it creates can push pH downward through a different pathway.
If the anion gap climbs above 12, clinicians look for either lactic acidosis or the possibility that the patient actually has both HHS and DKA occurring simultaneously.
The HHS-DKA Overlap
HHS and DKA are often taught as separate conditions, but in practice they frequently coexist. Up to 30% of patients with DKA also show features of HHS, and roughly a quarter of all hyperglycemic crises involve a combination of both. These overlap cases do involve significant metabolic acidosis, because ketone production is part of the picture.
Overlap is especially common in people with type 2 diabetes who are under severe physiological stress, such as a serious infection or a cardiac event. In these situations, stress hormones can overwhelm the small amount of residual insulin that would normally prevent ketogenesis. The result is extreme hyperglycemia and hyperosmolarity (the hallmarks of HHS) combined with ketoacid production and a low pH (the hallmarks of DKA). These mixed presentations tend to be more dangerous than either condition alone and require careful management of both the fluid deficit and the acidosis.
How the Numbers Compare
The lab values tell a clear story about where acidosis fits in each condition:
- pH in HHS: Usually above 7.3, often close to normal.
- pH in DKA: Typically between 6.8 and 7.2 at presentation.
- Bicarbonate in HHS: Generally 22 to 32 mmol/L, near the normal range.
- Bicarbonate in DKA: Significantly depressed, often well below 18 mEq/L.
- Anion gap in HHS: Variable, with about half of patients showing a mild elevation of 10 to 12.
- Anion gap in DKA: Consistently elevated above 12, often much higher.
Serum osmolality is where HHS stands apart. It must exceed 320 mOsm/kg for the diagnosis, compared to a normal range of 280 to 290. This extreme concentration of the blood is what drives the neurological symptoms, confusion, lethargy, and sometimes coma, that define HHS and make it dangerous in its own right, even without severe acidosis.
What Recovery Looks Like
Because the primary threat in HHS is dehydration and hyperosmolarity rather than acidosis, treatment centers on aggressive fluid replacement. Blood sugar in HHS typically normalizes within 8 to 10 hours, though clinicians deliberately keep the decline gradual to avoid neurological complications. The target is a glucose drop no faster than 90 to 120 mg/dL per hour, and osmolality should fall no more than 3 to 8 mOsm/kg per hour.
HHS is considered resolved when serum osmolality drops below 300 mOsm/kg, blood glucose falls below 250 mg/dL, urine output recovers, and mental status improves. If metabolic acidosis was present, whether from lactic acid or an overlap with DKA, it generally corrects as hydration restores tissue perfusion and kidney function returns to normal.