Rudi Hayden

Urologist

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Blood-tests Explained


Disclaimer:

This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

Salicylates


At a Glance

Why Get Tested?

To detect salicylate overdose, to help evaluate its severity and monitor its resolution; sometimes to monitor for an overdose if you regularly use prescription-strength salicylates

When to Get Tested?

When you have symptoms such as nausea, hyperventilation, ringing in the ears (tinnitus), or confusion that may be from taking too much aspirin or medications with salicylate ingredients; when it is suspected that an unconscious person or a child may have ingested a significant quantity of a salicylate, often as part of emergency and overdose testing; at regular intervals when monitoring an overdose; sometimes on a regular basis if you take a prescription salicylate for rheumatoid arthritis or other autoimmune disorder

Sample Required?

A blood sample drawn from a vein in your arm and/or a urine sample collected in a clean container

Test Preparation Needed?

None, but a healthcare practitioner may ask when you last took a salicylate and the amount taken. If you regularly take a prescription salicylate, the healthcare practitioner may want to collect blood just prior to your next dose (trough level). Tell your healthcare provider about any other medications you are taking.

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.

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).

The Test

How is it used?

This test is used to detect a high level of salicylate in the blood. It may be used to help diagnose an overdose (salicylate poisoning) or to monitor a person who takes a prescription-strength salicylate. The salicylate screening test may be used to screen for the presence of salicylate in urine.

If a person presents with symptoms that suggest aspirin or other salicylate poisoning, then a salicylate test may be ordered to help detect an overdose. If a healthcare practitioner is not sure what substance(s) a person may have ingested, then a salicylate test may be ordered along with other tests associated with emergency and overdose drug testing. The specific tests ordered will depend upon the person's symptoms and clinical signs.

In emergency situations, a salicylate test may be ordered with other tests, such as:

  • Arterial blood gases and electrolytes to evaluate oxygenation and acid-base balance
  • Blood urea nitrogen (BUN) and creatinine to evaluate kidney function
  • Urinalysis to monitor urine pH
  • Glucose to detect low blood sugar
  • Liver panel to evaluate liver function
  • Coagulation profile (PT, PTT, platelet count)

If a toxic concentration of salicylate is detected, then a series of salicylate tests (and some of the additional tests) may be ordered to determine whether the salicylate level has increased, or has peaked and started to drop. This information is used to help monitor the person's health status and help guide treatment.

A salicylate test may be used to monitor for an overdose if an individual takes regular prescribed doses of salicylate, such as to relieve the symptoms of rheumatoid arthritis, osteoarthritis, or lupus, or to treat rheumatic fever or Kawasaki disease. This may be especially appropriate if the person's other medications, or health status, have changed.

Monitoring is usually not necessary for individuals taking low-dose aspirin to lower the risk of blood clots (thrombosis), heart attack, or stroke (see Common Questions #3).

When is it ordered?

Salicylate testing may be ordered when it is suspected that someone has ingested a large amount of aspirin or other drugs containing salicylate. Usually, blood is drawn and tested at least 4 hours after last known ingestion. Results from tests done earlier than this are difficult to interpret.

Testing may be ordered when a person is experiencing symptoms associated with an acute or chronic salicylate overdose. Symptoms may include:

  • Nausea and vomiting
  • Hyperventilation (rapid breathing); less common in children
  • Ringing in the ears (tinnitus)
  • Sweating
  • Dizziness
  • Confusion

Additional symptoms that may develop include:

  • Headache
  • Agitation
  • Convulsions
  • Hallucinations
  • Rapid heart rate
  • Lethargy
  • Coma
  • Deafness
  • Overheating (hyperthermia), especially in young children with severe toxicity
  • Shortness of breath due to fluid accumulation in the lungs (pulmonary edema)
  • Bleeding (rare)

Testing may be done when people are suspected of wanting to harm themselves (suicidal tendencies).

When acute salicylate toxicity is detected, a healthcare practitioner may order the test as frequently as every 2-3 hours to monitor the resolution of the overdose.

When an individual is taking regular doses of a prescribed salicylate, a healthcare practitioner may order this test on a routine basis to check for an elevated level since toxicity may develop without obvious signs and symptoms. Someone with chronic salicylate toxicity may have symptoms that develop slowly and are nonspecific, such as confusion, nausea, rapid heart rate, and fever, and these may be seen with many other conditions. A salicylate test may be done to help determine whether salicylate toxicity is the cause of the symptoms.

When salicylate is not prescribed, then identifying chronic salicylate toxicity can sometimes be more challenging. After questioning a patient, a healthcare practitioner may order a salicylate level as part of a number of tests to help determine the cause of the person's symptoms.

What does the test result mean?

Normal therapeutic blood concentrations of aspirin and other salicylates depend upon what the drug is being used for, and blood levels must be interpreted in conjunction with a person's medical history and clinical signs and symptoms. Lower blood levels are sufficient for pain relief and to lower risk of blood clots, but higher levels may be required for managing inflammation in rheumatic conditions such as arthritis. At these higher levels, some side effects may become apparent.

The severity of signs and symptoms, and at what dose they emerge, depends upon the individual. Symptoms may be seen with blood levels that are generally considered to be therapeutic in inflammatory conditions, and they will tend to appear in many people at levels of about 10-30 mg/dL. In general, the severity of salicylate toxicity increases with increasing concentrations.

The table below summarizes some results that may be seen. These numbers typically represent a blood sample that is collected at least 4 hours after last dose:

Salicylate result reported as mg/dL Salicylate result reported as mcg/mL Result Interpretation
 2-10  20-100 Therapeutic level for pain relief (analgesia)
 10-30  100-300 Anti-inflammatory level; some symptoms of toxicity may appear, such as headache, tinnitus, vertigo
Greater than 30  Greater than 300  Considered toxic

Note: Some laboratories report salicylates as milligrams per deciliter (mg/dL) while others report micrograms per milliliter (mcg/mL). These two are different by a factor of 10.

Rising levels (when a person has not taken more salicylate) indicate that peak salicylate concentrations have not yet been reached. Falling salicylate levels tend to indicate that the excess is being eliminated by the body and that the overdose is resolving.

Salicylate toxicity is a serious condition that often requires hospitalization and careful monitoring. In severe cases, the acid-base imbalance that it causes can worsen over time, moving from respiratory alkalosis to metabolic acidosis depending on the period it was ingested, causing electrolyte imbalances, low blood glucose, and dehydration, and proceeding to convulsions, hallucinations, delirium, and coma.

Is there anything else I should know?

Be aware that many prescription and nonprescription medications contain salicylates in combination with other medications. Do not take more than one medication that contains salicylates at a time.

With large doses of salicylate, accumulations of the tablets (concretions, bezoars) may form in the digestive tract. This can cause the drug to continue to be absorbed for some time, increasing blood levels even though the person has not ingested more of the drug.

Aspirin should not be taken long-term without consulting a healthcare practitioner and should NEVER be given to children or teens unless directed by a healthcare provider. Aspirin should not be taken with alcoholic drinks as this can increase the risk of bleeding from the stomach. If aspirin overdose is suspected, seek medical attention for the affected person.

Salicylates, including aspirin, are included in the group of drugs called NSAIDs (nonsteroidal anti-inflammatory drugs).

A breakdown of muscle tissue (rhabdomyolysis) may sometimes occur as a complication of salicylate poisoning.