Final answer:
A trough level is the lowest concentration of a drug in the bloodstream, measured before the next dose, to ensure it stays above the minimum effective concentration and within the therapeutic range.
Step-by-step explanation:
A trough level refers to the lowest concentration of a drug in the bloodstream, typically measured just before the next dose is due. This measurement is significant because it helps to ensure that drug levels remain within the therapeutic range but above the minimum effective concentration. For example, when measuring the effectiveness of antibiotics, the trough level can indicate if the drug concentration has dropped below the required minimum inhibitory concentration (MIC) which would be ineffective at combating a bacterial infection. High-performance liquid chromatography (HPLC) or liquid chromatography-tandem mass spectrometry (LC-MS/MS) are often used to accurately determine these low concentration levels, especially when dealing with protein-bound drugs where only the free, unbound fraction is considered pharmacologically active.