An apparatus comprising a magnetic sensor device for detecting the presence of a target molecule in a stationary sample comprising: - a measurement sensor comprising a first moiety which covers a predetermined area of a substrate of the magnetic sensor device and which is a paratope which selectively binds to an epitope of the target molecule comprising a further epitope which is capable to bind with a paratope present on a first further moiety comprising a detectable label, the amount of the first further moiety binding to the first moiety being related to the amount of the target molecule present in said sample, the measurement sensor being configured to generate a first detection signal in response to detection of the detectable label in the first further moiety bound to the first moiety; and - a reference sensor comprising a second moiety which covers a predetermined further area of the substrate of the magnetic sensor device in a predetermined density and which selectively binds to a second further moiety comprising a further detectable label, the reference sensor being configured to generate a second detection signal in response to detection of the further detectable label in the second further moiety bound to the second moiety; - a signal processor configured to: during operation of the sensor device, determine whether the second detection signal falls within a signal value window corresponding to a predefined amount of the second further moiety binding to the second moiety, in response to the second detection signal falling outside the signal value window, generating a failure signal indicative of any of completion failure of the assay, insufficient fluid added to the test, incorrect preparation of the actuation protocol of the assay, lack of magnetic particles in the cartridge, failure of dry agents to disperse, unusually high clusters of magnetic particles, extensive reagent degradation, integrated reagent deterioration, or a damaged assay cartridge; and - a speaker or display which issues an audible or visible output in response to the failure signal, wherein the detectable label comprises a magnetic particle and the further detectable label comprises a further magnetic particle, and wherein the magnetic sensor device comprises a magnetic field generator for attracting the magnetic particle and the further magnetic particle to the respective sensors, wherein the area covered by the second moiety does not overlap with the area covered by the first moiety, and wherein the stationary sample is a sample that does not flow over the first area and the second area during detection measurement. - a measurement sensor comprising a first moiety which covers a predetermined area of a substrate of the magnetic sensor device and which is a paratope which selectively binds to an epitope of the target molecule comprising a further epitope which is capable to bind with a paratope present on a first further moiety comprising a detectable label, the amount of the first further moiety binding to the first moiety being related to the amount of the target molecule present in said sample, the measurement sensor being configured to generate a first detection signal in response to detection of the detectable label in the first further moiety bound to the first moiety; and - a reference sensor comprising a second moiety which covers a predetermined further area of the substrate of the magnetic sensor device in a predetermined density and which selectively binds to a second further moiety comprising a further detectable label, the reference sensor being configured to generate a second detection signal in response to detection of the further detectable label in the second further moiety bound to the second moiety; - a signal processor configured to: during operation of the sensor device, determine whether the second detection signal falls within a signal value window corresponding to a predefined amount of the second further moiety binding to the second moiety, in response to the second detection signal falling outside the signal value window, generating a failure signal indicative of any of completion failure of the assay, insufficient fluid added to the test, incorrect preparation of the actuation protocol of the assay, lack of magnetic particles in the cartridge, failure of dry agents to disperse, unusually high clusters of magnetic particles, extensive reagent degradation, integrated reagent deterioration, or a damaged assay cartridge; and during operation of the sensor device, determine whether the second detection signal falls within a signal value window corresponding to a predefined amount of the second further moiety binding to the second moiety, in response to the second detection signal falling outside the signal value window, generating a failure signal indicative of any of completion failure of the assay, insufficient fluid added to the test, incorrect preparation of the actuation protocol of the assay, lack of magnetic particles in the cartridge, failure of dry agents to disperse, unusually high clusters of magnetic particles, extensive reagent degradation, integrated reagent deterioration, or a damaged assay cartridge; and - a speaker or display which issues an audible or visible output in response to the failure signal,