Drug addiction and abuse are prevalent worldwide, and it is critical to quickly screen and accurately identify the suspected substance. Therefore, simplicity in sampling, speed in screening, and accuracy in quantification are keys to emergency treatment for a suspected abuse case. It is well-known that dried blood spot (DBS) is widely used in newborn screening for its less invasive sampling. DBS sample variation or severe alterations in the hematocrit may influence its results of concentration hence limits its use in quantitation. Inconsistent spotting also hampers the wider application of DBS in clinical practice. Compared to DBS, a micro plasma collection card may be an alternative for drug screening and quantification.
Micro plasma collection card is a sampling device that could automatically separate blood cells from plasma instantly after blood drawing. Specifically, only a drop of finger-prick blood is placed onto the test area of the card until the control spot turns red. After several minutes, the top layer of the card is peeled off. The lower layer is dried under ambient temperature. The volume (2.75±0.15 μL) of absorbed plasma is fixed so that the drug concentration in the dried plasma spot (DPS) can be accurately quantified. Besides, the DPS samples could be preserved at room temperature.
In our laboratory, we investigated the method for simultaneous screening and quantification of 10 drugs which included meperidine, fentanyl, morphine, oxycodone, tramadol, acetaminophen, heroin, ketamine, nimetazepam and methamphetamine. The quantification was done using DPS cards (prepared by Noviplex card, Beijing Bio Biotech Ltd.) by liquid chromatography tandem mass spectrometry method. A 50 μL of extraction mixture (Isopropanol: acetonitrile: ddH2O=3: 13: 22, including internal standard Acetaminophen-d4 and Fentanyl-d5) was used to extract drugs from the DPS cards. After extraction, abused drugs in the supernatant was separated by UPLC (Waters Acquity) and screened by TOF-MS IDA product ion scan (ABI QTOF5600+) based on retention time and exact mass acquired from molecular ions and fragment ions. The identified positive samples were confirmed by quantifying the precise concentration of each target drug by a multiple reaction monitoring mode with a quadrupole mass spectrometry (ABI 5500).
Plasma conversion factor was used to convert the concentration of the drugs on DPS to plasma. It was acquired by calculating the ratio of drug concentrations in DPS to those in wet plasma. The concentration conversion formula was: Wet plasma (ng·mL-1) =DPS (ng·mL-1) / plasma conversion factor. In our method, all the 10 drugs were well extracted from DPS with recoveries higher than 70%. For LC-QTOF MS screening method, the limit of detection was 10-50 ng/mL. For the LC-MS/MS quantification method, the accuracy was between 88.6-112.3% and precision was less than 10%; the linearity curve ranged from 10-1000 ng/mL. Plasma conversion factor of each drug was a constant value (from 0.0301 to 0.0597) when hematocrit was within reported reference range.
In conclusion, micro plasma collection card separates fixed plasma volume within a real separation time with a drop of finger-prick blood, therefore it is a convenient micro-sampling technique for screening 10 analgesics and narcotics in human plasma. In our study, we found that plasma conversion factor is not only compound-dependent, but is also affected by the component and volume of extraction solution.
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