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What Is a Therapeutic Level?

By Sandi Johnson
Updated Mar 03, 2024
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Therapeutic level can mean either of two concepts, both referring to the amount of pharmaceutical compound in the bloodstream of a patient. In scientific studies, therapeutic level, also known as the therapeutic ratio, refers to the difference between the amount of compound needed to produce a beneficial effect and how much will kill a patient. During routine medical treatment, therapeutic level refers to monitoring the amount of medication in a patient's bloodstream. The purpose of such testing is to ensure a pharmaceutical compound is present in the blood and that it is at a high enough concentration to offer effective treatment without causing harm.

Context is the most important clue in determining which definition applies, although professionals often use slightly different terminology to further prevent confusion. Typically, scientific studies refer to therapeutic level as the therapeutic ratio or the therapeutic index. Testing facilities and medical professionals typically refer to laboratory testing of therapeutic levels as therapeutic drug monitoring or therapeutic drug level. Specifically, therapeutic drug monitoring refers to the actual blood tests, while therapeutic drug level refers to the test results.

Although the two concepts differ slightly, both relate to the effective level of medication in the bloodstream of a patient. One merely focuses on the most extreme negative result, while the other focuses on avoiding harm to the patient during treatment. Scientists must first determine the therapeutic ratio so that doctors and testing facilities have the information necessary to determine initial dosing.

Ideally, doctors begin with a small initial dose of a compound, gradually increasing the dose until it becomes effective. Monitoring helps physicians to measure when the proper dosage is reach for a particular patient without reaching harmful levels. For example, medications used in the treatment of mental health disorders can be ineffective at low levels, but toxic at high levels. Finding the therapeutic level, or level at which the medication is effective without being toxic, requires monitoring of the concentrations in a patient's bloodstream.

Whether used in scientific research studies or routine patient care, therapeutic level is tested the same way. A small amount of blood is drawn and analyzed. Depending on the specific test and compound being tested, the exact amount of medication or other compound concentrated in the blood is measured first. Additional tests to check various organ functions may also be performed to compare against medication levels. Medications known to affect liver function, for example, may require a liver function test in additional to medication level tests.

Numerous factors can influence the correct therapeutic levels needed for various medications and compounds. Body size, ailment severity, and other medical conditions can change the recommended therapeutic level of a particular compound. Likewise, when used in scientific studies, the level of compound that produces death can also vary, depending on size and other factors. As such, recommended therapeutic levels are often represented as a range, such as 50-100 micrograms per milliliter or greater than .01 micromoles.

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Discussion Comments
By ZipLine — On Oct 13, 2014

The whole concept of therapeutic levels for medications doesn't seem very accurate to me. I think so because just last week, both my mother and I were given the same antibiotic and the same dose for a nasty cold. My mother was absolutely fine on it whereas I experienced lots of issues -- diarrhea, nausea, headaches. So obviously, the therapeutic level for a drug varies from person to person. It can't be pinned down in the lab.

By burcinc — On Oct 13, 2014

@ysmina-- No, I've never had blood tests for drug therapeutic level before either. But I don't think that it's something that's normally done. I suspect it may done if someone is given a medication that has a very narrow therapeutic window. Small increases in the dose can be very dangerous in these cases. So doctors may want to monitor drug levels in the drug very closely.

Normally, the medications we use have a wide enough therapeutic window that the doses we are prescribed won't become toxic. The worst case scenario is that there may be side effects. You should always inform your doctor when that happens because the dose probably needs to be lowered.

Side effects are actually very informative about the safe therapeutic levels of a drug. Lots of side effects and severe side effects show that the safe therapeutic level has been surpassed. The dose is too much for the patient. That's why patients should never adjust their dose on their own and they should never take a second dose.

By ysmina — On Oct 13, 2014

Has anyone here had their therapeutic levels tested? Is this something that ought to be done when using a medication regularly? I've never had this type of test before although I'm using two medications regularly for chronic conditions.

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