Cpk - indicates how well the INR reading variation (6 standard deviation spread, calculated using long term formulas) within the given date range actually fits within the therapeutic range for that patient. It takes into account if the average INR is not centered between the therapeutic range. It represents the actual situation, instead of the ideal situation (Cp), therefore Cpk will always be less than or equal to Cp. You can think of it as a penalty for being off from the middle of the therapeutic range.

Less than 1.0 = Poor, INR variation wider than therapeutic range
Equal to 1.0 = OK, INR variation equal to therapeutic range
Greater than 1.0 = Good, INR variation less than therapeutic range

Within the system, the patient capability reports will color code the Cp and Cpk values according to the following criteria (adjusted for healthcare).

Color	Cp/Cpk	Conclusion
Red	Less than 0.75	Too much variation in INR results to stay within therapeutic range over long term
Yellow	Between 0.75 and 1.0	Variation in INR results is adequate, but will not always be within therapeutic range over long term
Green	Greater than 1.0	Variation in INR results will stay within therapeutic range over long term