Polymer inclusion membranes (PIMs) have several important features, i.e., PIMs are dry and non-porous membranes, which can be prepared ahead of use and stored without noticeable deterioration in extraction performance. In this contribution, in-line coupling of microextractions across PIMs to a separation method for clinical purposes was demonstrated for the first time. Formate (the major metabolite in methanol poisoning) was determined in undiluted human serum and whole blood by capillary zone electrophoresis (CZE) with simultaneous capacitively coupled contactless conductivity detection (C(4)D) and UV-Vis detection. A purpose-made microextraction device with PIM was coupled to a commercial CZE instrument in order to ensure complete automation of the entire analytical procedure, i.e., of formate extraction, injection, CZE separation and quantification. PIMs for formate extractions consisted of 60% (w/w) cellulose triacetate as base polymer and 40% (w/w) Aliquat™ 336 as anion carrier. The method was characterized by good repeatability of peak areas (≤7.0%) and migration times (≤0.8%) and by good linearity of calibration curves (r(2) = 0.993-0.999). Limits of detection in various matrices ranged from 15 to 54 μM for C(4)D and from 200 to 635 μM for UV-Vis detection and were sufficiently low to clearly distinguish between endogenous and toxic levels of formate in healthy and methanol intoxicated individuals. In addition, PIMs proved that they may act as phase interfaces with excellent long-term stability since once prepared, they retained their extractions properties for, at least, two months of storage.