The invention relates to a method and apparatus for detecting undesired measurement conditions in a sample container. The method comprises measuring a fluorescent property of the sample container comprising a sample substrate with impregnated blood sample and incubation buffer to which the blood sample is to be eluted, and determining, based on temporal and/or spectral characteristics of the fluorescent property, whether the fluorescent property is characteristic to a sample container comprising a sample substrate and incubation buffer under said undesired measurement conditions or to a sample container suitable for optical measurement of analyte contained in the sample. Thus undesired measurement condition can be a floating sample substrate or a foreign body in the sample container. By means of the invention, reliability of neonatal screening, for example, can be increased.