Metformin, Contrast and a Mislabeled Acidosis
A 58-year-old man with type 2 diabetes (BMI 31) is admitted for a CT angiogram. His regular medicines are metformin 1 g twice daily, ramipril 10 mg daily and atorvastatin 20 mg nightly. Baseline serum creatinine is 168 µmol/L and eGFR is 38 mL/min/1.73 m². He receives iohexol contrast and remains on metformin because the ward chart is not reviewed. Two days later he is drowsy with deep, sighing respiration. Arterial blood gas on room air: pH 7.27, pCO2 26 mmHg, HCO3 13 mmol/L. Electrolytes: Na 138, K 4.9, Cl 101 mmol/L; glucose 12 mmol/L; lactate 5.2 mmol/L; beta-hydroxybutyrate 0.4 mmol/L; urea 14 mmol/L; creatinine 210 µmol/L. The admitting doctor documents 'diabetic ketoacidosis, start sliding-scale insulin' and asks you to review the case before the insulin infusion begins.
1. Calculate the anion gap and the delta ratio. Does the measured pCO2 fit the metabolic acidosis? What acid-base pattern do these numbers define?
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Anion gap = Na − (Cl + HCO3) = 138 − (101 + 13) = 24 mmol/L. With a normal gap of about 10–12 mmol/L, the gap is raised by roughly 12–14 mmol/L. Delta ratio = ΔAG / ΔHCO3 = (24 − 12) / (24 − 13) = 12 / 11 ≈ 1.1. A ratio near 1 indicates a pure high-anion-gap acidosis, without a substantial additional metabolic alkalosis or non-gap acidosis.
Winter's formula: expected pCO2 = 1.5 × HCO3 + 8 ± 2 = 1.5 × 13 + 8 ± 2 = 27.5 ± 2 mmHg. The measured pCO2 of 26 mmHg lies inside that range, so respiratory compensation is appropriate. There is no additional respiratory acidosis or alkalosis.
The pattern is therefore a single high-anion-gap metabolic acidosis with appropriate respiratory compensation. This matters because it rules out a mixed disorder before the team attributes everything to ketones, and it forces the lactate and renal failure to be explained.
2. The team's label is diabetic ketoacidosis. Using the glucose, ketone and lactate values, argue for or against that label, and state the most likely diagnosis with two supporting features.
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DKA is unlikely. The glucose is only 12 mmol/L, and beta-hydroxybutyrate is 0.4 mmol/L, which is too low to explain a gap of 24 mmol/L. Euglycaemic DKA can occur with SGLT2 inhibitors, but this man is not on one. Starvation ketosis would not produce this degree of acidosis with a normal glucose.
The dominant anion is lactate, at 5.2 mmol/L. The history provides two hits: metformin accumulation after contrast in a patient with eGFR 38, and worsening renal function (creatinine rising from 168 to 210 µmol/L). The working diagnosis is metformin-associated lactic acidosis, triggered by contrast and acute kidney injury.
Supporting features are the high lactate with low ketones and the temporal link to contrast while metformin was continued. A serum metformin concentration would support the diagnosis but should not delay treatment. Insulin infusion for presumed DKA would be inappropriate and could cause hypoglycaemia without addressing the lactate.
3. State your immediate management priorities and the criteria you would use before metformin is ever restarted.
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Stop metformin immediately. Give isotonic crystalloid cautiously, monitor urine output and treat the contrast-associated kidney injury. Because pH is 7.27, bicarbonate is not routinely indicated; reserve it for severe acidaemia, commonly pH below 7.1, and only after senior discussion. Urgent renal replacement therapy is the definitive way to clear metformin and lactate if there is worsening kidney injury, refractory acidosis or haemodynamic instability. The eGFR of 38 and rising creatinine make early nephrology involvement necessary.
Before any restart, renal function must stabilise and the eGFR must be above the relevant threshold, commonly 30 mL/min/1.73 m², with the precipitant resolved and a documented review. If metformin is restarted, use a reduced dose and counsel about sick-day rules. Pause the insulin sliding scale until glucose and ketones are interpreted correctly.