The invention relates to a sample processing apparatus comprising a holder for a microtiter plate comprising a plurality of microwells, optical measurement unit for measuring optical responses of samples dosed to the microwells, and a computing unit configured to analyze the optical responses in order to detect dosing failures in said plurality of microwells, and if a dosing failure has been detected in one or more of the microwells, to communicate the existence of the dosing failure to a user of the apparatus through signaling means or to store data indicative of the dosing failure to data storage means for further use. In particular, the invention relates to detecting dosing failures in before, during and after a PCR process.