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 Sometimes the appropriate time for taking a TDM sample is immediately prior to the next dose when the drug concentration is at its minimum (trough concentration). Sometimes, a sample collected immediately after drug administration (peak concentration) may be required. Evaluation of the clinical significance of the test result therefore depends on knowledge of the time when the sample was drawn.
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Your health & safety is important to usPlease note that face masks are mandatoryat all times at all of our locations to ensure the health and safety of all customers and employees. Please inform staff if you are unable to wear a mask due to a medical condition. Thank you for your cooperation!Indications for TestingNot all medications require TDM, especially when the drug has a wide therapeutic index or low risk for severe adverse effects. TDM is utilized for drugs with a:
TDM is also used to identify drug-drug or food-drug interactions and to monitor patient compliance with treatment, status during decontamination or detoxification, and changes in drug concentrations related to aging, pregnancy, or clinical status. A list of example drug-drug interactions that may be identified by TDM can be found on the U.S. Food and Drug Administration’s Drug Development and Drug Interactions web page. TDM should be performed once a drug has reached steady-state concentration. Specimen SelectionTherapeutic and toxic ranges are typically established for serum, plasma, and whole blood specimens. Urine should not be utilized for TDM. Serum/Plasma and Whole BloodClinical signs and symptoms of effective drug treatment, ineffective drug treatment, and toxicity may correlate with drug and/or metabolite concentrations in serum, plasma, and/or whole blood. Serum, plasma, and whole blood specimens are also appropriate for patients on dialysis, for suspected cases of malabsorption (eg, due to gastric bypass), and for evaluating other aspects of an individual patient’s pharmacokinetics. Whole BloodWhole blood specimens are used for TDM of select drugs such as immunosuppressive drugs (eg, cyclosporine A, tacrolimus, everolimus, sirolimus, and thiopurine drugs) due to drug accumulation in red blood cells (RBCs). Oral Fluid (Saliva)Oral fluid drug concentrations tend to correlate with serum/plasma concentrations. The window of drug detection is approximately 1-2 days after drug use; therefore, oral fluid can be used to detect recent drug exposure. Therapeutic ranges are not well established in oral fluid. UrineUrine drug and metabolite concentrations do not correlate with signs and symptoms of drug therapy or toxicity; therefore, urine is not a recommended specimen type for TDM. Urine drug concentrations should also not be used to extrapolate the dose that was administered. Timing of Specimen CollectionTherapeutic ranges are typically established at timed blood collections after steady-state concentrations have been reached (generally 5-7 half-lives after initiation of or change in dosing):
Commonly Used Testing StrategyQuantitative testing for TDM may be performed by immunoassay, high performance liquid chromatography (HPLC), or mass spectrometry. Drug results that are reported as less than the assay cutoff should be interpreted as “not detected.” Frequently Asked QuestionsWhat is the definition of half-life?The half-life of a drug refers to the time it takes for 50% of the drug to be eliminated from blood. What is the definition of steady-state concentration?Steady-state concentration occurs when the rate of drug administration is equal to the rate of elimination. Generally, steady-state concentration can be achieved after an individual has consistently received the drug for the duration of 5-7 half-lives (eg, if a drug has a half-life of 24 hours and is administered once a day, then steady-state concentration can be achieved after 5-7 days of drug administration). What is the window of detection of drugs in blood, serum, or plasma specimens?In general, the window of detection in blood, serum, and plasma is 1-2 days after drug administration. The window of detection for drugs is dependent on several factors, including the following:
Can gel separator tubes be used for toxicology testing?Gel separator tubes are not recommended for testing in toxicology. Drugs that are lipid soluble may be absorbed into the gel and may cause a falsely low drug result. When is the best time to collect samples for peak level?To assess peak levels, the time for drawing depends on the route of administration: Oral: One hour after drug is taken (assumes a half-life of > two hours) IV: 15-30 minutes after injection/infusion. Intramuscular (IM): 30 minutes - one hour after injection.
Why does the time for the peak and trough blood levels of certain drugs matter?Peak and trough levels are drawn to determine a drug's concentration within the system. They help determine if a drug is in a toxic range or if the dosage of the medication needs to be increased. It is important to know which medications need to be monitored and what the signs and symptoms of toxicity are.
When should trough and peak levels be drawn for therapeutic drug monitoring?Usually blood samples are collected at the end of the dosage interval (trough level). For antibiotics administered intravenously, peak concentrations are also measured at 30 min following infusion cessation.
What is time to peak for a drug?Definition: The time it takes for a drug to reach the maximum concentration (Cmax) after administration of a drug that needs to be absorbed (e.g. an oral drug). Tmax is governed by the rate of drug absorption and the rate of drug elimination.
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What is generally the optimal time to collect a sample for a peak drug level?
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