Acetylsalicylic Acid

Acetylsalicylic acid (ASA) is a common analgesic, anti-inflammatory agent widely available as aspirin as an over the counter medication. More than 100 medications contain aspirin. Children’s aspirin contains 81 mg ASA and adult aspirin contains either 325 mg ASA (regular strength) or 500 mg ASA (extra-strength). 

Generally, therapeutic doses of aspirin are rapidly absorbed from the GI tract and peak serum levels are reached by 6 hours after ingestion.  Absorption may be delayed in a massive overdose because of concretion of multiple tablets in the stomach. In this situation, serum  levels may continue to rise for 24 hours after ingestion.  

Measurement of serum peak levels of acetysalicylic acid can be used to assure therapeutic levels or confirm intoxication. Five to seven days are required to reach steady state therapeutic levels. Therapeutic peak  serum levels are; 

Enteric coated tables are formulated to resist the acidic pH of the stomach and to dissolve in the alkaline fluid of the small intestine.  Drug release is primarily a function of gastric emptying time. A serum salicylate level measured at 6 hours may not represent the peak level, which may be delayed by many hours.  

Some patients may experience toxicity at levels above 30 mg/dL. Tinnitus usually occurs as serum levels approach 30 mg/dL. Values  >40 mg/dL are often considered critical, but serious toxicity usually occurs at levels  >50 mg/dL. Wintergreen topical solutions contain methyl salicylate and ingestion of as little as 1 tsp can cause systemic toxicity.  

The clinical signs and symptoms of salicylate toxicity include tinnitus, decreased auditory acuity, tachypnea, hyperpyrexia, sweating, vomiting, gastric pain, and confusion. Patients develop respiratory alkalosis initially, followed by metabolic acidosis. Respiratory alkalosis is due to direct salicylate stimulation of the respiratory center in the medulla oblongata.  Metabolic acidosis and hyperpyrexia are caused by uncoupling of oxidative phosphorylation and production of lactic acid, acetoacetic acid and hydroxybutyrate.  An increased demand for glucose, causes hyperglycemia initially due to increased glycolysis.  Later, hypoglycemic ensues as glucose stores are depleted.  High salicylate levels cause pulmonary edema by increasing the permeability of pulmonary capillaries to protein and fluid. Chronic toxicity is more insidious than acute toxicity, particularly in the elderly.  

Specimen requirement is one SST tube of blood. Therapeutic peak levels should be drawn 2 hours after the last dose.  After an overdose, a specimen should be drawn 6 hours after ingestion and then every 4 hours until a downward trend in salicylate level is established.

Reference

Arif H, Aggarwal S. Salicylic Acid (Aspirin) [Updated 2023 Jul 5]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK519032/