The blood plasma radiometabolite analysis (RMA) is crucial procedure for quantitative analysis of positron emission tomography (PET) radioligands with no original reference region in the brain. Blood sampling and separating plasma for RMA has been a challenge especially with small animal PET research. This work aimed to overcome this problem by presenting a semi-automated lead shielded microextraction by packed sorbents (SLS-MEPS). The capability of SLS-MEPS was evaluated for RMA of [C]SMW139 as a PET radioligand in rat plasma samples. [C]SMW139 is a potential radioligand with high receptor binding for P2XR in vivo. The validation method was individually performed for [C]SMW139 and [C]SMW139 with reversed phase high performance liquid chromatography (HPLC) utilizing radio- and UV-detector, respectively. The limits of detection (LOD) and quantification (LOQ) [C]SMW139 in rat plasma were 3 and 10 Becquerel (Bq), respectively. The standard calibration curve was obtained for [C]SMW139 within the concentration range 10-15000 Bq with over 0.995 coefficients for all assays. In addition, the capability of SLS-MEPS in combination with HPLC-UV for extraction of [C]SMW139 in rat plasma was also challenged and the obtained results showed a proper linearity (20-2000 μg mL), accuracy and repeatability.