11 Oct 2026 · Case #036
Salicylate poisoning
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Diagnosis
Salicylate poisoning
First-line treatment
Sodium bicarbonate for urinary alkalinisation, with hemodialysis if severe
How the diagnosis is reached
The blood gas is the whole case. A pH of 7.44 looks reassuring until the numbers underneath are read together: the pCO₂ is far too low and so is the bicarbonate, and neither explains the other. Salicylate stimulates the respiratory centre directly, producing a primary respiratory alkalosis, and at the same time uncouples oxidative phosphorylation, producing a high anion gap metabolic acidosis. The two push the pH in opposite directions and hold it near normal, which is why a mixed picture in a hyperventilating patient is close to diagnostic. A pure metabolic acidosis would never drive the pCO₂ to 2.6, and a pure respiratory alkalosis would not consume bicarbonate. Uncoupling also explains the fever and the drenching sweat, both of which invite a mistaken diagnosis of sepsis. Tinnitus and muffled hearing are early and characteristic. The rest of the screen closes the other causes of a raised anion gap: normal glucose and ketones exclude the ketoacidoses, a normal osmolar gap excludes ethylene glycol and methanol, and the paracetamol level matters because co-ingestion is common and silent. Finally, levels still climbing at eight hours show absorption is continuing, often from a mass of tablets in the stomach, so one reassuring early level never excludes toxicity. Treatment then follows from the chemistry. Salicylate is a weak acid, so raising the urine pH traps it as an ion in the tubule and stops it being reabsorbed: her urine pH of 5.4 is the number to move, with intravenous sodium bicarbonate. Potassium goes in alongside, since a depleted kidney exchanges potassium for hydrogen and quietly undoes the alkalinisation. Hemodialysis is held for a level above roughly 700 mg/L, renal failure, refractory acidosis, seizures or altered consciousness. The one thing not to do is intubate her for that respiratory rate of 34: the hyperventilation is all that holds her pH at 7.44, and a ventilator failing to match it lets the pH fall off a cliff.
Key clinical clues
- Vomiting and deep, rapid breathing after an overdose she will not confirm
- Tinnitus and muffled hearing, early and characteristic of salicylate
- A near-normal pH concealing both a low pCO₂ and a low bicarbonate, so respiratory alkalosis and metabolic acidosis together
- Raised anion gap with normal glucose, ketones and osmolar gap, excluding ketoacidosis and the toxic alcohols
- Levels still rising at eight hours, showing absorption is not finished
The case in full
Presenting complaint
Patient history
A 19-year-old woman is brought in by her flatmate after four hours of vomiting. She is breathing fast and deeply, sweating and agitated. She was found upset in her room after an exam result and will not say whether she has taken anything.
Symptoms
Symptoms
She says her ears are ringing and that sounds seem muffled. The nausea continues. There is no abdominal pain, no diarrhoea and no chest pain.
Examination and observations
Vital signs
She is alert but restless, flushed and sweating heavily. There is no muscle rigidity, no clonus and no focal neurological sign. Her pupils are mid-sized and reactive.
| Parameter | Value | Reference |
|---|---|---|
| Temperature | 38.2 °CRef 36.1–37.5 | 36.1–37.5 |
| Respiratory rate | 34 /minRef 12–20 | 12–20 |
| Heart rate | 118 bpmRef 60–100 | 60–100 |
| Blood pressure | 108/64 mmHg | – |
| SpO₂ (room air) | 99 %Ref 95–100 | 95–100 |
Arterial blood gas
Laboratory results
A gas is taken on room air.
| Parameter | Value | Reference |
|---|---|---|
| pH | 7.44Ref 7.35–7.45 | 7.35–7.45 |
| pCO₂ | 2.6 kPaRef 4.7–6.0 | 4.7–6.0 |
| Bicarbonate | 14 mmol/LRef 22–26 | 22–26 |
| Base excess | -9Ref -2 to +2 | -2 to +2 |
| Anion gap | 22Ref 8–16 | 8–16 |
| Lactate | 2.8 mmol/LRef <2.0 | <2.0 |
Further bloods
Laboratory results
The rest of the screen comes back.
| Parameter | Value | Reference |
|---|---|---|
| Glucose | 5.2 mmol/LRef 3.9–7.8 | 3.9–7.8 |
| Ketones | 0.3 mmol/LRef <0.6 | <0.6 |
| Creatinine | 78 µmol/LRef 45–90 | 45–90 |
| Osmolar gap | 6 mOsm/kgRef <10 | <10 |
| Paracetamol level | UndetectableRef Undetectable | Undetectable |
| Ethanol | Not detectedRef Not detected | Not detected |
Serial drug levels
Laboratory results
A specific drug level is sent on arrival and repeated twice. Her urine pH is 5.4.