The Z prime value (Z´)

Classical assay parameters like signal to blank (S/B) and signal to noise (S/N) are well known in laboratories and are often used to determine the dynamic range of an assay.

Dr EJ Dell Dr EJ Dell

But sometimes these parameters are not sufficient enough. In this case the Z´(pronounced Z prime) value comes to mind. The Z´ value takes into account four (4) parameters: the means (µ) and the standard deviations (σ) of the positive (plateau) and the negative (baseline) controls1. The controls are equivalent to the upper and lower limit of the assay.

Formula Z Prime Value

The formula above shows that a good Z´ value is only obtained when the means of both controls are significantly different from each other, and the standard deviations of both controls are as low as possible. Thus, the Z´value is not only a measurement for a good or bad assay, but it is also a measurement for a good or bad instrument.

An assay and instrument are considered excellent when the Z´value is between 0.5 and 1. Sometimes a Z´ value < 0.5 is also acceptable if there is no way for improvement, i.e. you are using the best instrument and there is no better assay format. If the Z´value is < 0, it suggests that the positive and negative control values are too close to each other and that the assay is not working.

For more information on assays showing very good Z´values in combination with the plate readers from BMG LABTECH please visit our application notes center.




  1. JH Zhang 1, TD Chung, KR Oldenburg. A Simple Statistical Parameter for Use in Evaluation and Validation of High Throughput Screening Assays. J Biomol Screen. 1999;4(2):67-73.

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