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Favipiravir is a broad-spectrum inhibitor of viral RNA polymerase. It is currently used as a possible treatment for coronavirus disease 2019 (COVID-19). Pre-clinical or clinical trials of favipiravir require robust, sensitive, and accurate bioanalytical methods for quantitation of favipiravir levels. Recently, several studies have been reported about developing a validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for measuring favipiravir levels. However, these methods were validated predominantly for plasma samples, electrospray ionization was operated only in negative or positive mode, and clinical application of these methods has not been applied for patients with COVID-19. This study aimed was to develop a validated LC-MS/MS method for the measurement of favipiravir levels in positive and negative electrospray ionization mode and to perform a pilot study in patients with COVID-19 receiving favipiravir to demonstrate the applicability of this method in biological samples. Simple protein precipitation was used for the extraction of favipiravir from the desired matrix. Favipiravir levels were quantitated using MS / MS with an electrospray ionization source in positive and negative multiple reaction monitoring (MRM) mode. The chromatographic detection was performed on a reverse-phase Phenomenex C18 column (50 mm x 4.6 mm, 5µm, 100 Å) with gradient elution using 0.1% formic acid in water and 0.1% formic acid in methanol as mobile phase. The method was linear over the concentration ranges of 0.048–50 µg/mL (in negative ionization mode) and 0.062-50 µg/mL (in positive ionization mode) with a correlation coefficient (r 2 ) better than 0.998. The total run time was 3.5 min. The intra-assay and inter-assay %CV values were less than 7.2% and 8.0%, respectively. A simple, rapid and robust LC-MS / MS method was developed for the measurement of favipiravir and validation studies were performed. The validated method was successfully applied for drug level measurement in COVID-19 patients receiving favipiravir.

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ObjectivesOur aim was to validate a mass spectrometric creatinine method and compare this method with Jaffe and enzymatic serum creatinine methods.Methods90 samples were included. The levels were classified into three groups according to serum creatinine results as Group 1: Lower (n=30) (0.16–0.59 mg/dL), Group 2: Normal (n=30) (0.62–1.18 mg/dL) and Group 3: Higher (n=30) (1.33–3.88 mg/dL). Jaffe and enzymatic creatinine measurements were performed on the Beckman Coulter AU5800 autoanalyzer.ResultsSerum creatinine was linear from 0.039 up to 10 mg/dL, CV and bias values were ranged between 1.9–3.8% and 2–15%. Correlation coefficients were 0.990 (95% confidence interval 0.984–0.993), 0.992 (95% confidence interval 0.988–0.995) and 0.994 (95% confidence interval 0.991–0.996) for LC-MS/MS-Enzymatic, LC-MS/MS-Jaffe and Enzymatic-Jaffe, respectively.ConclusionsAlthough, Jaffe method for serum creatinine measurement is still much more practical and cheap, so in use for routine practice, tandem mass spectrometric detection of serum creatinine can be used as an accurate and specific method for verification of discordant clinical results, existence of possible interferences and serum levels under 0.5 mg/dL creatinine results such as pediatric or pregnant populations.

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