<p>Furosemide is a diuretic drug used to increase urine flow in order to reduce the amount of salt and water in the body. It is commonly utilized to treat preterm infants with chronic lung disease of prematurity. There is a need for a simple and reliable quantitation of furosemide in human urine. We have developed and validated an ultra-high performance liquid chromatography-tandem mass spectrometry method for furosemide quantitation in human urine with an assay range of 0.100-50.0&nbsp;μg/ml. Sample preparation involved solid-phase extraction with 10&nbsp;μl of urine. Intra-day accuracies and precisions for the quality control samples were 94.5-106 and 1.86-10.2%, respectively, while inter-day accuracies and precision were 99.2-102 and 3.38-7.41%, respectively. Recovery for furosemide had an average of 23.8%, with an average matrix effect of 101%. Furosemide was stable in human urine under the assay conditions. Stability for furosemide was shown at 1&nbsp;week (room temperature, 4, -20 and -78°C), 6&nbsp;months (-78°C), and through three freeze-thaw cycles. This robust assay demonstrates accurate and precise quantitation of furosemide in a small volume (10&nbsp;μl) of human urine. It is currently being implemented in an ongoing pediatric clinical study.</p>

<p><strong>PURPOSE: </strong>With the increasing prevalence of multi-drug resistant organisms, therapeutic drug monitoring (TDM) has become a common tool for assuring the safety and efficacy of antimicrobial drugs at higher doses. Microsampling techniques, including dried blood spotting (DBS) and volumetric absorptive microsampling (VAMS), are attractive tools for TDM and pediatric clinical research. For microsampling techniques to be a useful tool for TDM, it is necessary to establish the blood-plasma correlation and the therapeutic window of antimicrobial drugs in the blood.</p>

<p><strong>METHODS: </strong>DBS involves the collection of small volumes of blood (30 - 50 µL per spot) on a filter paper, while VAMS allows the accurate and precise collection of a fixed volume of blood (10-30 µL) with microsampling devices. One of the major advantages of VAMS is that it reduces or eliminates the volumetric blood hematocrit (HCT) bias associated with DBS. Liquid chromatography with tandem mass spectrometry (LC-MS/MS) is a powerful tool for the accurate quantification of antimicrobial drugs from small volumes of blood specimens.</p>

<p><strong>RESULTS: </strong>This review summarizes the recent LC-MS/MS assays that have employed DBS and VAMS approaches for quantifying antimicrobial drugs. Sample collection, extraction, validation outcomes, including the inter- and intra-assay accuracy and precision, recovery, stability, and matrix effect, as well as the clinical application of these assays and their potential as tools of TDM are discussed herein.</p>

<p><strong>CONCLUSION: </strong>Microsampling techniques, such as VAMS, provide an alternative approach to traditional plasma sample collection for TDM.</p>

<p>Vancomycin is a commonly used antibiotic, which requires therapeutic drug monitoring to ensure optimal treatment. Microsampling assays are attractive tools for pediatric clinical research and therapeutic drug monitoring. A LC-MS/MS&nbsp;method for the quantification of vancomycin in human whole blood employing volumetric absorptive microsampling (VAMS) devices (20&nbsp;μl) was developed and validated. Vancomycin was stable in human whole blood VAMS under assay conditions. Stability for vancomycin was established for at least 160&nbsp;days as dried microsamples at -78°C. This method is currently being utilized for the quantitation of vancomycin in whole blood VAMS for an ongoing pediatric clinical study and representative clinical data are reported.</p>

<p>Cefepime is a fourth-generation cephalosporin antibiotic with an extended spectrum of activity against many Gram-positive and Gram-negative bacteria. There is a growing need to develop sensitive, small volume assays, along with less invasive sample collection to facilitate pediatric pharmacokinetic clinical trials and therapeutic drug monitoring. The volumetric absorptive microsampling (VAMS™) approach provides an accurate and precise collection of a fixed volume of blood (10 μL), reducing or eliminating the volumetric blood hematocrit assay-bias associated with the dried blood spotting technique. We developed a high-performance liquid chromatographic method with tandem mass spectrometry detection for quantification of cefepime. Sample extraction from VAMS™ devices, followed by reversed-phase chromatographic separation and selective detection using tandem mass spectrometry with a 4 min runtime per sample was employed. Standard curves were linear between 0.1-100 μg/mL for cefepime. Intra- and inter-day accuracies were within 95.4-113% and precision (CV) was &lt; 15 % based on a 3-day validation study. Recoveries ranged from 40.8 to 62.1% and the matrix effect was within 89.5-96.7% for cefepime. Cefepime was stable in human whole blood under assay conditions (3 h at room temperature, 24 h in autosampler post-extraction). Cefepime was also stable for at least 1 week (7 days) at 4 °C, 1 month (39 days) at -20 °C and 3 months (91 days) at -78 °C as dried microsamples. This assay provides an efficient quantitation of cefepime and was successfully implemented for the analysis of whole blood microsamples in a pediatric clinical trial.</p>