In the field of Plasma Medicine gas discharges and biology are inevitably linked. However, as new sources and developments evolve there is an increasing need for assays screening for plasma effects on cells. In turn, by assessing certain properties eukaryotic cells can be regarded as biosensors for plasma properties e.g. reactive oxygen species. We here present assays which were designed to help deciphering how plasma influences cells. For this, we used an atmospheric pressure argon plasma jet and human blood cells and investigated cellular viability and oxidation. We show that care must be taken when designing and interpreting plasma experiments involving cells and provide a hypothesis to dismantle the mechanism of plasma mediated cytotoxicity. These may be useful for other researchers who may use different plasma sources and cell types but are in need to characterizing their individual plasma-cell interaction.