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The Test Sample
What is being tested?
Salicylates are a group of drugs, including aspirin, available as both prescription and non-prescription (over-the-counter) medications. They are often used to relieve pain and inflammation, to reduce fever, and to prevent excessive clotting. Salicylate testing measures the concentration in the blood to detect and/or monitor an overdose (salicylate poisoning). Sometimes, a screening test (Trinder's test) performed on a urine sample may lead to suspicion of the presence of salicylate in the blood.
Aspirin is the most common salicylate, but there are others, including methyl salicylate, which is found as oil of wintergreen in some muscle ache creams. Methyl salicylate creams and topical forms of aspirin are absorbed into the body through the skin. After oral forms of salicylates are ingested, they are converted to salicylic acid, absorbed in the stomach and small intestine and transferred into the blood. They are metabolized by the liver and eliminated from the body in the urine within a few hours.
With single, normal doses, blood concentrations typically peak in about 2 hours, but this may be delayed for 12 hours or more with "enteric-coated" or "sustained-release" preparations. If too much salicylate is ingested (acute toxicity with a large ingestion) or too much is taken over time (chronic toxicity), and/or if the body's ability to remove the salicylates is impaired, then signs and symptoms associated with toxicity will begin to emerge. The ability to remove salicylates efficiently is affected by the body's blood and urine pH (acidity/alkalinity) and by kidney and liver function.
Over-the counter salicylates are used as needed or regularly to reduce pain, fever, and inflammation. Low doses of aspirin may be taken on a regular basis to reduce the chance of inappropriate blood clotting (thrombosis), heart attack, or stroke in people who have a high risk. Aspirin may also be used to reduce the risk of complications in someone who is having a heart attack or who recently experienced one. Aspirin is also used in patients with myeloproliferative neoplasms (e.g., polycythemia vera, essential thrombocythemia). Uncommonly, prescription-strength aspirin may be used to relieve symptoms of rheumatoid arthritis (RA) and osteoarthritis as well as symptoms of autoimmune disorders such as lupus.
When used with care and following dosing directions, therapeutic doses of salicylates are safe and effective for most adults. Excessive amounts of salicylates, however, can be toxic, affecting breathing and disrupting the body's acid-base balance. In severe cases, toxicity can cause convulsions, coma, and even death.
Adults can run into trouble with salicylates when they unknowingly combine multiple products that contain them. Many over-the-counter medications include a salicylate as one of the ingredients. An individual who is unaware of the ingredients can mistakenly take more than one of these drugs together, resulting in a cumulative effect and a high amount of ingested salicylate. Widespread and easy availability of salicylate-containing medications can also mislead some to think that it is very safe and that "more is better." An overdose can occur when someone has pain that is not relieved with an initial dose and, unaware of the danger, takes more than the recommended dose and/or takes additional doses too frequently. In the elderly, these scenarios can especially be a problem because underlying conditions and general health status can decrease the ability to eliminate salicylates.
Aspirin is not recommended for use in children and adolescents because of the risk of developing Reye syndrome, a disease characterized by acute brain damage and liver dysfunction that can be fatal. Though youth are not routinely given aspirin, they may become poisoned through accidental or intentional ingestion. Topical creams that contain methyl salicylate or other salicylates are of special concern as they contain very high doses.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm. A random urine sample, or a catheter urine sample from an unconscious patient may also be obtained.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed, but a healthcare practitioner may ask when a salicylate was last ingested and the amount taken. To accurately interpret results, blood samples are usually drawn at least 4 hours after ingestion. If a person regularly takes a prescription salicylate, the healthcare provider may want to collect blood just prior to the next dose (trough level).