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Case Study 5: Re-calibration Does Not Correct Non-linear Response

The following data were obtained from routine calibration verification / linearity testing for digoxin:

Level	X	Rep 1	Rep 2	Rep 3
1	1.0	0	0	0
2	2.0	0.20	0.21	0.21
3	3.0	1.22	1.18	1.34
4	4.0	2.24	2.33	2.33
5	5.0	3.17	3.25	3.28

Results from the Maine Standards Company data reduction service were as follows:

			+/-	%	+/-	%
X	Target	Mean	Diff	Diff	Limit	Limit
1.0	-0.139	0.000	**0.138	100.0%	0.100	N/A
2.0	0.484	0.207	**0.277	57.2%	0.100	N/A
3.0	1.108	1.247	0.139	**12.5%	0.111	N/A
4.0	1.731	2.300	0.569	**32.9%	0.173	N/A
5.0	2.354	3.233	0.879	**37.3%	0.235	N/A

The results showed signficant non-linearity for the method and did not meet the laboratory’s acceptance criteria. Also, The same lot of VALIDATE^® material was tested on two other instrument systems and all levels passed on both systems. The laboratory contacted Maine Standards Company to discuss the results. Maine Standards Company Technical Support staff explained that according to the CLIA regulations, paragraph section 493.1255 (a)(3), the recommended actions for failure of calibration verification / linearity testing is to re-calibrate the test.

After several unsuccessful attempts to re-calibrate the digoxin assay, the laboratory requested a service call from the instrument system manufacturer. During the service call, a probe alignment issue was discovered and corrected. After successful re-calibration, the laboratory performed calibration verification / linearity testing again for digoxin with the same lot of VALIDATE^® material originally used. The data obtained were as follows:

Level	X	Rep 1	Rep 2	Rep 3
1	1.0	0.38	0.29	0.33
2	2.0	1.48	1.59	1.54
3	3.0	2.93	3.00	2.96
4	4.0	3.96	3.90	3.93
5	5.0	5.88	5.42	5.65

			+/-	%	+/-	%
X	Target	Mean	Diff	Diff	Limit	Limit
1.0	0.296	0.333	0.037	12.5%	0.100	N/A
2.0	1.611	1.537	0.074	4.6%	0.161	10%
3.0	2.926	2.963	0.037	1.3%	0.293	10%
4.0	4.241	3.930	0.311	7.3%	0.424	10%
5.0	5.556	5.650	0.094	1.7%	0.556	10%

In this case, routine calibration verification / linearity testing demonstrated an instrument system issue that required a service call. In most cases, re-calibration of an assay corrects a calibration verification / linearity testing failure. However, occasionally, re-calibration does not correct the problem and the total instrument system must be checked.

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