The present invention relates to a method and apparatus for determining a correction factor for detecting hemolysis or for correcting the effect of hemolysis in measuring hematocrit. The invention further relates to an extracorporeal blood treatment apparatus comprising a device for detecting hemolysis. The present invention is based on the implementation of two different optical measurement methods for determining the hematocrit value, in which hemolysis or correction factors are established based on the hematocrit values detected by the two measurement methods. Hematocrit values detected by different measurement methods have been shown to indicate that they are affected at different intensities by the increase in plasma free hemoglobin concentration resulting from hemolysis. The preferred embodiment of the present invention assumes reflection measurement as the first measurement method and transmission measurement as the second measurement method. Consequently, detecting hemolysis or establishing a correction factor is achieved based on forming a difference in hematocrit values in the reflection and transmission measurements. The apparatus of the present invention includes a control and analysis unit configured to be established based on values when hemolysis or correction factors are detected for hematocrit values according to the first and second measurement methods. [Selection] Figure 1
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