A 35-year-old woman with a history of recurrent episodes of severe lactic acidosis (pH 7.1, lactate 8.5 mmol/L) triggered by fasting or minor infections is referred for investigation. Baseline serum lactate is 2.8 mmol/L. To confirm a defect in the Cori cycle and identify whether the problem lies in lactate production or hepatic clearance, which test is most appropriate as the initial investigative step?

Explanation

## Diagnostic Approach to Cori Cycle Dysfunction ### Why Fasting Lactate–Pyruvate Profile is the First-Line Investigation **Key Point:** The fasting lactate and pyruvate levels, combined with glucose and insulin, allow non-invasive differentiation between: - **Overproduction of lactate** (high lactate, high pyruvate, low glucose → suggests muscle/mitochondrial disease) - **Impaired hepatic clearance** (high lactate, low pyruvate, low glucose → suggests hepatic gluconeogenic defect) ### Interpretation Framework | Finding | Interpretation | Likely Defect | |---------|-----------------|---------------| | ↑ Lactate, ↑ Pyruvate, ↓ Glucose | Lactate overproduction | Mitochondrial myopathy, GSD type I | | ↑ Lactate, ↓ Pyruvate, ↓ Glucose | Impaired hepatic clearance | Gluconeogenic enzyme deficiency (PEPCK, FBPase) | | ↑ Lactate, ↑ Pyruvate, ↑ Glucose | Hepatic lactate uptake intact but glucose overproduction | Rare; suggests substrate cycling | ### Why This Answers the Clinical Question In **Cori cycle dysfunction**, the fasting state (when the cycle is most active) reveals: - Inability of liver to clear lactate → persistent elevation - Failure to convert lactate → glucose → hypoglycemia - Lactate-to-pyruvate ratio >20 suggests impaired hepatic lactate oxidation **High-Yield:** Fasting lactate >2.5 mmol/L is abnormal and warrants further investigation. The lactate-to-pyruvate ratio is more specific than absolute lactate level for identifying the metabolic block. ### Clinical Pearl Patients with **glucokinase deficiency** or **PEPCK deficiency** present with severe fasting lactic acidosis because the liver cannot convert lactate to glucose, forcing continued lactate accumulation. Feeding rapidly corrects the acidosis by suppressing lactate production and activating glycogenesis. **Mnemonic:** **FLAP** — **F**asting **L**actate, **A**rterial **P**yruvate ratio distinguishes production from clearance defects.