A 28-year-old woman with known type 1 diabetes mellitus presents to the emergency department with a 2-day history of polyuria, polydipsia, and progressive lethargy. On examination, she has Kussmaul respirations and fruity-smelling breath. Capillary blood glucose is 520 mg/dL. Which investigation is most appropriate to confirm the diagnosis of diabetic ketoacidosis and assess severity?

Explanation

## Diagnostic Confirmation of DKA **Key Point:** DKA diagnosis requires demonstration of metabolic acidosis (pH < 7.35, HCO₃⁻ < 18 mEq/L) combined with ketonemia/ketonuria and hyperglycemia. Venous blood gas (VBG) is the single best initial test in resource-limited settings because it provides pH and bicarbonate directly. **High-Yield:** While arterial blood gas (ABG) is more accurate for PaO₂ and PaCO₂, venous blood gas is equally reliable for pH and HCO₃⁻ assessment and is preferred in emergency settings for speed and patient comfort. Serum bicarbonate alone is insufficient without pH confirmation. ### Why VBG + Serum Bicarbonate? | Parameter | Role in DKA Diagnosis | |-----------|----------------------| | **Venous pH** | Confirms acidemia (< 7.35) | | **HCO₃⁻** | Quantifies severity (mild: 15–18, moderate: 10–14, severe: < 10 mEq/L) | | **Anion gap** | Calculated from electrolytes; typically elevated (> 12) | | **Glucose** | Usually 250–800 mg/dL; not diagnostic alone | | **Urine/serum ketones** | Supportive but not required if acidosis + hyperglycemia proven | **Clinical Pearl:** In DKA, serum bicarbonate is the most sensitive marker of acidosis severity and guides insulin/fluid titration. A patient with pH 7.30 and HCO₃⁻ 12 has moderate DKA requiring ICU-level care. **Mnemonic — DKA Diagnostic Triad: GAK** - **G**lucose > 250 mg/dL - **A**cidosis (pH < 7.35) - **K**etones (serum/urine positive) ### Investigation Sequence 1. **Immediate:** VBG (pH, HCO₃⁻), serum glucose, electrolytes (Na⁺, K⁺, Cl⁻), BUN/creatinine 2. **Concurrent:** Serum/urine ketones (β-hydroxybutyrate preferred over acetoacetate) 3. **Secondary:** ABG if respiratory compromise suspected; osmolality if HHS differential considered [cite:Harrison 21e Ch 396]