The Z prime value (Z´)

January 06, 2012

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. Edward Dell | BMG LABTECH
Dr. EJ Dell
PhD, Sales Manager Northwest
BMG LABTECH USA

But sometimes these parameters are not efficient enough. In this case the Z´(speak Z prime) value come to the fore. Z´ takes into account 4 parameters: the means (µ) and the standard deviations (σ) of the positive and the negative control. The controls are equivalent to the upper and lower limit of the assay.

The formula above shows that a good Z´ value is only obtained when the means of both controls are strongly different to each other and the standard deviations of both controls are as low as possible. Because of that the Z´value does not only show if an assay is good or bad, it is also a measure for a good or bad instrumentation.


An excellent assay and instrumentation is given when the Z´value is between 0.5 and 1. Sometimes Z´< 0.5 is also accepted. Z´< 0 indicates that the values of the positive and negative control are too close to each other – the assay is not working.


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

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