Drug concentration is typically measured to monitor the amount of a drug present in a patient's system, ensuring safe and effective treatment. This process is vital in therapeutic drug monitoring (TDM). The sample used for measurement depends on the drug and the clinical objective.
Common Sample Types for Drug Concentration Measurement
| Sample Type | Description | When Used |
|---|---|---|
| Serum or Plasma | The liquid part of blood after clotting (serum) or after the blood cells are removed (plasma). | Most commonly used for TDM of most drugs. |
| Whole Blood | Untreated blood containing all components such as red and white blood cells, platelets and plasma. | More clinically relevant for certain drugs, particularly immunosuppressants. |
Methods of Measurement
1. Laboratory Analysis
* **Chromatography:** Techniques like High-Performance Liquid Chromatography (HPLC) and Gas Chromatography (GC) separate the drug from other substances in the sample, allowing its concentration to be measured.
* **Spectrometry:** Methods like Mass Spectrometry (MS) measure the mass-to-charge ratio of ions, which can identify and quantify the drug.
* **Immunoassays:** These use antibodies to detect and quantify the drug based on antigen-antibody interactions.2. Point-of-Care Testing
* **Rapid Tests:** Some devices are designed to quickly measure drug levels at the bedside or in a clinic.
* **Biosensors:** Specialized devices are able to measure specific drug levels in small samples.Example: Immunosuppressants and Whole Blood Measurement
The reference highlights that for drugs like immunosuppressants, whole blood concentrations are often more clinically relevant. This is because these drugs are taken up by blood cells which impact the therapeutic effect and the measurement of drug concentration. Therefore, using serum or plasma may not accurately reflect the drug's activity.
- Reason: Many immunosuppressants accumulate within blood cells, and these intracellular drug levels correlate better with clinical response.
- Implication: TDM for these drugs often requires whole blood samples to ensure that patients receive proper dosing.
Key Considerations:
- Timing: When blood is drawn for testing in relation to the timing of drug administration is crucial for accurate interpretation.
- Patient Factors: Patient-specific factors such as age, kidney or liver function, and drug interactions can affect drug concentration in the body, requiring customized monitoring.
- Assay Specificity: The method employed needs to be specific to the drug being measured, preventing inaccurate results from similar substances.
By employing these techniques, healthcare providers can accurately measure drug concentrations, leading to better treatment outcomes, improved patient safety, and reduced adverse effects.
