Not all drug levels need to be monitored. These tests are used to monitor blood concentrations of particular drugs that have a narrow dose range in which the drug is effective but not toxic. In addition, some drugs require monitoring because the amount of drug administered does not correlate well with the amount of drug that reaches the bloodstream. Sometimes, the way that a particular drug is absorbed and metabolised can vary from person to person, or the physical or health status of a person can affect the drug level in the blood.
Through years of testing, the optimum ‘therapeutic ranges’ for effective drug concentrations in the blood have been determined. Within these concentration ranges, most people will be effectively treated without excessive side effects or symptoms of toxicity. The drug dosage necessary to reach this concentration must be determined for each individual. When a person starts on a monitored drug (or returns to it after an absence), the healthcare professional adjusts the dose upwards and tests blood concentrations frequently until the appropriate steady level is achieved. If someone's levels are too high, the healthcare professional will lower the dosage. Often, each different dosage level will take a short period of time to stabilise, so these corrections up and down may take place over a few days or weeks. It is important that people work closely with their healthcare professional during this process and not make their own adjustments or stop taking their medication. Abrupt changes can sometimes worsen conditions and cause acute symptoms.
When are they requested?
Concentrations of monitored drugs are often tested frequently when a person is first put on a drug. Once a person's results are in the therapeutic range and his or her clinical signs indicate that the treatment is appropriate, then the healthcare professional may monitor the drug at less frequent intervals as needed to ensure that the drug concentration stays in the therapeutic range. The frequency of testing required will depend on the drug and on the needs of the patient. If treatment does not appear to be fully effective, or if the person has excessive side effects or signs of toxicity, then the healthcare professional will request testing aimed at adjusting the drug dosage and maintaining levels within the therapeutic range. Monitoring may also take place during illness or other changed circumstances such as pregnancy. Sometimes, the healthcare professional may need to re-evaluate the use of a specific medication and consider switching to another type of drug to better fit the person's condition.
The timing of blood collection is an important part of therapeutic drug monitoring. After a person takes a dose of drug, the amount in the blood rises for a short period of time, reaches a peak and then begins to fall, usually reaching its lowest level (the ‘trough level’) just before the next dose. For the drug to be most effective, peak levels should be below toxic concentrations and trough levels should remain in the therapeutic range. Through experience and studies, healthcare professionals know when to expect peaks and troughs and will request blood sample collections as either trough levels (usually collected just before the next dose), peak levels (for which timing varies depending on the drug), or sometimes as a randomly timed level. Consistent and accurate interpretation of the results depends on the timing of sample collection, so it is important to be clear when your blood sample needs to be taken in relation to when you took the drug. If you are unable to take your medication or have blood collected at the appropriate time, then you should talk to your healthcare professional before the sample is collected.