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Therapeutic Drug Monitoring and Clinical Toxicology
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Applicability of Volumetric Absorptive Microsampling in Pediatric Populations

Constantinos Pistos

This month we are publishing a recent contribution to the Compass about the applicability of Volumetric Absorptive Microsampling in pediatric populations, by Constantinos Pistos, on behalf of the Alternative Sampling Strategies Committee. This application promises to overcome some of the challenges of performing TDM in children, however plasma to whole blood relationships must be demonstrated, and often new therapeutic windows must be established for clinical applicability. Read on!


Constantinos Pistos
Associate Professor of Toxicology
Analytical Toxicology Laboratory, Department of Chemistry
West Chester University, PA, USA


Performing Therapeutic Drug Monitoring (TDM) in children and young adults is technically and ethically challenging [1]. The recent progress which has been made in the field of microsampling techniques have led to assays which appear to overcome the barriers of these challenges and allow the quantification of drugs in smaller volume (< 50 µL) of biological fluid and in some cases improvement of the accuracy of results when comparing to plasma concentrations. A comprehensive review of the progress on microsampling techniques has been recently published [2]. Dried Blood Spot (DBS) and Volumetric Absorptive Microsampling (VAMS™) are the two predominant techniques which are used for TDM. Although DBS is the most widely used technique, it presents some disadvantages [3] which has led to the development of accurate volume dried blood collection devices such as the VAMS [4]. However, only during the last years, a number of studies have been published reporting the use of VAMS in children.

In one of the recent studies, the VAMS method was applied for quantifying cefepime in 7 clinical samples obtained from one subject [5]. In vitro comparison of the concentrations of cefepime in VAMS, wet whole blood, and plasma samples at three quality control (QC) concentrations showed that the plasma concentrations were a 2-fold higher compared to the whole blood and VAMS. The blood to plasma partitioning ratios for cefepime ranged from 0.524 – 0.578 and VAMS to plasma partitioning ratio from 0.529 – 0.570. In-vivo, 50 mg/kg cefepime were administered intravenously over 30 minutes and the results showed that the concentration of cefepime measured from the finger stick VAMS (36.6 µg/mL) and arterial VAMS (38.8 µg/mL) samples were similar.

In another antibiotic drug study, Moorthy et al. [6] developed and validated a VAMS assay for the quantification of vancomycin in pediatric samples by LC–MS/MS and evaluated in-vitro the concentration of vancomycin in whole blood, plasma and whole blood VAMS. This validated method was clinically evaluated using 3 pediatric subjects. The whole blood and whole blood VAMS had similar vancomycin concentrations at the three quality control levels. However, vancomycin concentrations in human plasma showed a 30–55% increase compared with the whole blood and VAMS. The ratios for blood to plasma ranged from 0.689 to 0.707 and the VAMS to plasma ratios from 0.644 to 0.767. The mean VAMS to plasma ratio for vancomycin was 0.718. The arterial/venous whole blood VAMS vancomycin concentration was divided by the mean VAMS to plasma partitioning ratio (0.718) to provide an estimate of plasma vancomycin concentration, which was found to be consistent with the measured plasma vancomycin concentration from the central lab of the hospital in two out of three cases (0.72, 1.04, 0.78).

During the last years the interest in cannabis-based therapies has been increased. In one of two reported studies involving cannabinoids in children, the concentrations of delta-9-tetrahydrocannabinol (Δ9-THC), cannabidiol (CBD), and cannabinol (CBN) were compared in VAMS, blood, and plasma samples through a validated LC-MS/MS method [7]. The in-vitro VAMS to plasma partitioning ratios for Δ9-THC, CBD, and CBN of the quality control samples were 0.589–0.702, 0.696– 0.827, and 0.659–0.763, respectively. The authors applied the validated method for clinical validation on five samples from one pediatric subject who was administered 10 mg edible CBD product at home. The authors concentration of CBD in plasma was estimated using the validated blood to plasma ratios. The levels of Δ9-THC and CBN were not quantifiable in these samples. In the second study [8], VAMS was evaluated for the quantification of CBD blood levels to be used in clinical practice in 5 pediatric subjects with Dravet syndrome treated with CBD oral solution (15 mg/kg/day Epidyolex®). CBD concentrations were determined in capillary and venous blood obtained by micro-sampling and were compared with CBD concentration in plasma. Linear regression analysis showed a correlation between CBD concentrations measured on capillary blood sampled by VAMS and did not differ from those measured by venous VAMS (R2 > 0.98) and plasma from venipuncture (R2 > 0.93). Furthermore, a midazolam study from hospitalized children (n=56), showed calculated concentrations of midazolam and 1-OH midazolam to have a very strong correlation (R2 = 0.97) between the plasma, and VAMS data [9]. The authors noted though that the comparison data is a limited dataset which only includes the initial earliest set of study samples analyzed. The average blood/plasma ratio for Valproic Acid in twenty- one pediatric left-over blood samples, was found to be 0.66. In the same study, the average blood/plasma ratio of carbamazepine in 56 VAMS samples was 1.21 and for phenobarbital was 0.93 [10]. Table 1 summarizes the VAMS blood/plasma ratios of the above studies.

Microsampling provides an alternative approach to traditional plasma sampling in pediatric clinical research and TDM. Enhanced sensitivity of modern instruments provides the capabilities to quantify low concentrations of drugs from small volume of clinical samples. VAMS provides several benefits to patients including the collection of small volumes of blood, reducing the risk for infection and alleviating anxiety with venipuncture blood draws. However, most drugs require establishment of a new therapeutic window utilizing whole blood VAMS to be implemented successfully for TDM in children.

Table 1. Comparison of VAMS blood/plasma ratio using in-vitro assays and clinical samples from pediatric patients.

Analyte In-vitro whole blood/
plasma ratio
In-vitro whole blood VAMS™/
plasma ratio
Clinical whole blood VAMS™/
plasma ratio or correlation (R2)
Cefepime 0.524-0.578 0.529-0.570 - 5
Vancomycin 0.689-0.707 0.649-0.767 0.72-1.04a 6
- 7
CBD - - Venous VAMS/plasma: 0.98-1.37a
Capillary VAMS/plasma: 0.72-1.29a
Midazolam - - 0.97b 9
Valproic Acid
- 10

a Blood VAMS™/plasma ratio b Coefficient of determination (R2)


[1] Mulugeta YL, Zajicek A, Barrett J, Sachs HC, McCune S, Sinha V, Yao L. Development of drug therapies for newborns and children: the scientific and regulatory imperatives. Pediatr. Clin. North Am. 64(6), 1185–1196 (2017).

[2] Delahaye L, Veenhof H, Koch BCP, Alffenaar J-W C, Linden R, Stove C. Alternative Sampling Devices to Collect Dried Blood Microsamples: State- of-the-Art. Ther. Drug Monit. 43:310–321 (2021).

[3] de Vries R, Barfield M, van de Merbel N, Schmid B, Siethoff C, Ortiz J, Verheij E, van Baar B, Cobb Z, White S, Timmerman P. The effect of haematocrit on bioanalysis of DBS: results from the EBF DBS-microsampling consortium. Bioanalysis. 5(17), 2147–2160 (2013).

[4] Denniff P, Spooner N. Volumetric absorptive microsampling: a dried sample collection technique for quantitative bioanalysis. Anal. Chem. 86(16), 8489–8495 (2014).

[5] Moorthy GS, Vedar C, Zane NR, Downes KJ, Prodell JL, DiLiberto MA, Zuppa AF. Development and validation of a volumetric absorptive microsampling- liquid chromatography mass spectrometry method for the analysis of cefepime in human whole blood: application to pediatric pharmacokinetic study. J. Pharm. Biomed. Anal. 179:113002 (2020).

[6] Moorthy GS, Downes KJ, Vedar C, Zuppa AF. A whole blood microsampling assay for vancomycin: development, validation and application for pediatric clinical study. Bioanalysis.12(18), 1295–1310 (2020).

[7] Moorthya GS, Vedara C, DiLiberto MA. Zuppa AF. A patient-centric liquid chromatography-tandem mass spectrometry microsampling assay for analysis of cannabinoids in human whole blood: Application to pediatric pharmacokinetic study. J. Chrom. B. 1130–1131 (2019) 121828.

[8] Dubois S, Marchese F, Pigliasco F, Barco S, Tripodi G, Lomonaco T, Lattanzi S, Russo E, Cangemi G and Striano P. A Volumetric Absorptive Microsampling Technique to Monitor Cannabidiol Levels in Epilepsy Patients. Front. Pharmacol. 11:582286. doi: 10.3389/fphar.2020.582286 (2020).

[9] Abu-Rabie P, Neupane B, Spooner N, Rudge J, Denniff P, Mulla H, Pandya H; Validation of methods for determining pediatric midazolam using wet whole blood and volumetric absorptive microsampling. Bioanalysis. 11(19), 1737–1754 (2019).

[10] Velghe S, Delahaye L, Ogwang R, Hotterbeekx A, Colebunders R, Mandro M, Idro R, Stove CP. Dried Blood Microsampling-Based Therapeutic Drug Monitoring of Antiepileptic Drugs in Children With Nodding Syndrome and Epilepsy in Uganda and the Democratic Republic of the Congo. Ther. Drug Monit. 42:481–490 (2020).

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