A 62-year-old man with type 2 diabetes mellitus presents with a 5-day history of progressive confusion, polyuria, and weakness. On examination, he is drowsy but arousable, with dry mucous membranes and skin turgor loss. Capillary blood glucose is 580 mg/dL. Arterial blood gas shows pH 7.32 with HCO₃⁻ 16 mEq/L. Serum osmolality is 335 mOsm/kg. Which investigation would be most helpful in distinguishing HHS from DKA in this clinical scenario?

Explanation

## Distinguishing HHS from DKA: Role of Ketones **Key Point:** Serum beta-hydroxybutyrate is the MOST DISCRIMINATORY investigation to differentiate HHS from DKA. In HHS, ketones are ABSENT or MINIMAL (<3 mmol/L); in DKA, they are MARKEDLY ELEVATED (>3 mmol/L). This is the defining biochemical distinction between the two conditions. ### Clinical Overlap and Diagnostic Challenge Both DKA and HHS present with: - Severe hyperglycemia (>250 mg/dL) - Hyperosmolality (>315 mOsm/kg) - Metabolic acidosis (pH <7.30, HCO₃⁻ <15) - Altered mental status - Dehydration However, the **presence or absence of significant ketosis** is the KEY DIFFERENTIATOR. ### Comparison Table: DKA vs HHS | Feature | DKA | HHS | |---------|-----|-----| | **Glucose** | 250–600 mg/dL | 600–1200 mg/dL (higher) | | **pH** | <7.30 (severe acidosis) | >7.30 (mild/no acidosis) | | **HCO₃⁻** | <15 mEq/L | >15 mEq/L | | **Anion gap** | >12 (high) | Normal or <12 | | **Serum ketones** | **>3 mmol/L (ELEVATED)** | **<3 mmol/L (ABSENT/MINIMAL)** | | **Osmolality** | 310–320 mOsm/kg | >320 mOsm/kg (higher) | | **Type 1 DM** | Common | Rare | | **Type 2 DM** | Less common | Common | **High-Yield:** In the case presented, the patient has: - Mild acidosis (pH 7.32, HCO₃⁻ 16) — NOT severe - Anion gap likely normal or low (not calculated but suggested by mild acidosis) - Marked hyperglycemia (580 mg/dL) and hyperosmolality (335 mOsm/kg) - Type 2 diabetes (HHS risk factor) These features suggest **HHS rather than DKA**. **Serum beta-hydroxybutyrate will be LOW (<3 mmol/L), confirming HHS.** **Clinical Pearl:** HHS is a "metabolic state without ketoacidosis" — insulin is present in sufficient quantity to suppress lipolysis and ketone production, but is insufficient to lower glucose. DKA occurs when insulin is severely deficient, allowing unopposed lipolysis and ketone overproduction. **Mnemonic: HHS vs DKA** — **"K for Ketones"** - **DKA**: **K**etones **HIGH** (>3 mmol/L) - **HHS**: **K**etones **LOW** (<3 mmol/L) ### Diagnostic Algorithm ```mermaid flowchart TD A[Hyperglycemic crisis: glucose >250, altered mental status]:::outcome A --> B[Check ABG/VBG: pH and HCO3-]:::action B --> C{Severe acidosis?<br/>pH <7.30, HCO3- <15}:::decision C -->|Yes| D[Check serum beta-hydroxybutyrate]:::action C -->|No| E[Likely HHS: check ketones to confirm]:::action D --> F{Ketones >3 mmol/L?}:::decision F -->|Yes| G[DKA CONFIRMED]:::outcome F -->|No| H[HHS with mild acidosis]:::outcome E --> I{Ketones <3 mmol/L?}:::decision I -->|Yes| J[HHS CONFIRMED]:::outcome I -->|No| K[Mixed picture or euglycemic DKA]:::outcome ``` **Warning:** Do NOT rely on anion gap alone — HHS can present with a NORMAL anion gap metabolic acidosis (hyperchloremic acidosis from volume depletion), making it indistinguishable from DKA without ketone measurement.