Salicylate Unit Converter
Salicylate Unit Converter
Convert a salicylate concentration between mg/L, µmol/L, mg/dL and mmol/L — and read the number the way a toxicologist does, which is never on its own.
Salicylate converter
Mass ⇄ molarSalicylate 320 mg/L
Formula and conversion factors
mmol/L = mg/L ÷ 138.12 · mg/dL = mg/L ÷ 10
- 7.24008
- 1 mg/L ÷ 138.12 g/mol, expressed in µmol/L. A concentration of 300 mg/L is 2,172 µmol/L, or 2.17 mmol/L
- mg/L against mg/dL
- a factor of ten, and both units are in routine use — 300 mg/L is 30 mg/dL. Reading one as the other is the single commonest error with this assay
- 138.12
- the molar mass of salicylic acid. Assays report the salicylate anion, but the acid molar mass is the reporting convention
- which salicylate
- the assay measures salicylate however it arrived — aspirin, methyl salicylate in topical or herbal preparations, bismuth subsalicylate — and does not distinguish them
Worked example
Salicylate 320 mg/L
320 × 7.24008 = 2,317 µmol/L
320 ÷ 10 = 32.0 mg/dL
320 ÷ 138.12 = 2.32 mmol/L
Above the therapeutic anti-inflammatory range of 150–300 mg/L
That fact alone says almost nothing about how unwell this patient is — the arterial pH, the clinical state and the acute-or-chronic history say far more
The same concentration in every unit in use
| mg/L | mg/dL | µmol/L | mmol/L | Conventionally described as |
|---|---|---|---|---|
| 150 | 15.0 | 1,086 | 1.09 | Lower end of the anti-inflammatory therapeutic range |
| 300 | 30.0 | 2,172 | 2.17 | Upper end of therapeutic; clinical toxicity is usually seen above this |
| 500 | 50.0 | 3,620 | 3.62 | Established toxicity in an acute adult overdose |
| 700 | 70.0 | 5,068 | 5.07 | The concentration above which most adult deaths have occurred |
| 1,000 | 100.0 | 7,240 | 7.24 | Severe acute poisoning |
Three things that matter more than the number
| Why | What follows | |
|---|---|---|
| Arterial pH and the acid-base picture | Acidaemia increases the un-ionised fraction that crosses into the brain, so the same concentration is far more dangerous at a low pH. A rising pCO₂ or a falling pH signals exhaustion of the respiratory compensation | Blood gas with the level, and repeated with it |
| Acute or chronic, and how old the patient is | Chronic poisoning in an older person is missed as sepsis, delirium or pneumonia, and produces severe illness at concentrations a young acute overdose tolerates | The history decides how the number is read, not the other way round |
| Whether the concentration is still rising | Enteric-coated aspirin and tablet bezoars absorb slowly and erratically; a first level taken early can be falsely reassuring and continue rising for 12 hours or more | Serial concentrations, typically every two to four hours, until they are clearly falling |
Four units, a tenfold trap, and a number that does not decide anything
Salicylate is reported in mg/L in most of the United Kingdom, in mg/dL in the United States, and in µmol/L or mmol/L wherever molar units are preferred. The molar conversion uses the molar mass of salicylic acid, 138.12 g/mol, so 1 mg/L is 7.24 µmol/L and a concentration of 300 mg/L is 2,172 µmol/L or 2.17 mmol/L. The dangerous conversion is the simplest one: mg/L and mg/dL differ by a factor of ten, both are in routine use, and 300 mg/L read as 300 mg/dL is a tenfold overestimate.
The concentration alone does not decide management, and treating it as though it does is the central error in salicylate poisoning. Arterial pH matters more. Salicylate crosses into the central nervous system in its un-ionised form, so acidaemia drives more of the same total concentration into the brain and makes it far more dangerous. Whether the exposure was acute or chronic matters more still: chronic poisoning, typically in an older person taking salicylate regularly, is repeatedly mistaken for sepsis, delirium or pneumonia, and produces severe illness at concentrations a young patient after a single overdose would tolerate.
The concentration also has to be repeated. Absorption after overdose is slow and erratic — enteric-coated preparations delay it, and salicylate tablets can form a bezoar in the stomach that continues to release drug for many hours — so a first concentration taken early may be low and still climbing. Serial measurements, conventionally every two to four hours until they are clearly falling, are what distinguish a resolving exposure from one that has not yet peaked. A single reassuring level early after ingestion is not a safe basis for discharge.
One point belongs in every account of this poisoning. The tachypnoea of a salicylate-poisoned patient is not distress to be relieved; it is a compensatory respiratory alkalosis holding the pH up and the drug out of the brain. Intubation and mechanical ventilation take that compensation away, and the apnoeic period around induction plus any ventilation that fails to match the patient’s own minute volume can precipitate an abrupt fall in pH and cardiac arrest. It is a decision for senior clinicians in consultation with a poisons service, with a plan for the pH before the drugs are drawn up.
Frequently asked questions
How do I convert salicylate from mg/L to µmol/L?
Multiply by 7.24, using the molar mass of salicylic acid of 138.12 g/mol. A concentration of 300 mg/L is 2,172 µmol/L, which is 2.17 mmol/L. Divide the mg/L figure by 10 for mg/dL.
What is the therapeutic salicylate range?
Roughly 150 to 300 mg/L — 15 to 30 mg/dL, or 1.1 to 2.2 mmol/L — for anti-inflammatory or antirheumatic dosing. Clinical toxicity is usually seen above 300 mg/L, and most adult deaths have occurred above 700 mg/L, but those are descriptions of concentration and not thresholds for any particular action.
Why does a salicylate level need repeating?
Because absorption after overdose is slow and unpredictable. Enteric-coated tablets delay it and salicylate can form a gastric bezoar that keeps releasing drug for many hours, so a first level taken early can be falsely reassuring and continue to rise. Serial levels every two to four hours until they are clearly falling are the standard approach.
Why is the salicylate level a poor guide to how sick the patient is?
Because severity depends on the arterial pH, the clinical state and whether the exposure was acute or chronic at least as much as on the concentration. A chronically poisoned older person can be critically unwell at a level a young acute overdose patient tolerates. The number is interpreted by a clinician alongside the patient.
Why is intubating a salicylate-poisoned patient dangerous?
Their hyperventilation is a protective compensatory respiratory alkalosis that keeps the pH up and the drug out of the brain. Losing it around induction, or ventilating below the patient’s own minute volume, can cause an abrupt fall in pH and cardiac arrest. It is a senior decision taken with a poisons service.
Related calculators
References
- Dargan PI, Wallace CI, Jones AL. An evidence based flowchart to guide the management of acute salicylate (aspirin) overdose. Emerg Med J. 2002;19(3):206–209.
- Juurlink DN, Gosselin S, Kielstein JT, et al. Extracorporeal treatment for salicylate poisoning: systematic review and recommendations from the EXTRIP workgroup. Ann Emerg Med. 2015;66(2):165–181.
- Chyka PA, Erdman AR, Christianson G, et al. Salicylate poisoning: an evidence-based consensus guideline for out-of-hospital management. Clin Toxicol (Phila). 2007;45(2):95–131.