Posted by Sten Westgard, MS
We got an excellent question the other day via email:
I have heard the term "Within QC" and "Across QC"used, but what do these refer to specifically andwhere can I find more information about what ismeant by those terms? I was not able to find thisinformation, but laboratory leadership staff saidthat "Within QC" referred to assessing multi-rules"within QC level and across QC runs", and that"Across Qc" referred to assessing multi-rules"looking at both QC levels, can be within same runor back-to-back runs".A lab has the following multi-rules; "Within QC"1:3s, 1 QC result outside 3sd; 2:2s, 2 consecutiveQC results outside 2sd on the same side of the mean;4s, 2 consecutive Qc results differ by more than4sd; and 1:2s, 1 Qc result outside 2sd and within 3.(1:2s is used as a warning rule, the others asrejection rules). The rules for "Across Qc" are asfollows; 2:2s, 2 consecutive Qc results (1 eachmultiple levels) are outside of 2sd; and 4s, 2 Qcresults (one of each multiple levels) are >4sdapart. These are both rejection rules. Thesemulti-rules are used to assess all tests in achemistry lab; the majority of tests are assessedwith 2 levels of Qc, a few use 3 levels of Qc.The situation arose where QC results on one day fora cancer antigen were the following:Day 1A:Level 1 -Within 2sd, acceptableLevel 2 -1:2s, run was accepted as only the warningrule 1:2s was encountered.The next day the results were as follows:Day 2A:Level 1 -1:2sLevel 2 -Within 2sdLeadership said run should not be accepted,violating the "across" 2:2s rule.However, leadership said the inverse situation wouldhave been acceptable as *consecutive* data pointsdid not violate the "across" 2:2s rule, i.e.
Day 1B:Level 1 1:2sLevel 2 within 2sdDay 2B:Level 1 within 2sdLevel 2 1:2sIn the A group, because Level 2 is outside of 2s,and the very next data point (Level 1 from the nextday) is also out 2s, the run is unacceptable andshould be rejected. In Group B, since consecutivedata points are okay the run is acceptable.Is this a correct approach? Is it correct to rejectgroup A (Day 1A and 2A) and not reject group B (Day1B and 2B)? Do these multi-rules as outlined andimplemented detect some unacceptable variation ingroup A that does not exist in group B? Thank youfor any clarification.
So what's the answer? Are scenarios A and B fundamentally different? More after the jump.
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Posted by Sten Westgard, MS
Here's a question that came in about setting the control limits (or range) for a test:
"for some assays we're using this formula: actual SD * 3 and then divided by 2 plus or minus the mean is this acceptable or not because when we use that give us abit wider range than using the mean plus minus 2SD."
When we asked for an example, we got this data:
Manufacturer Data: SD = 22.5, Mean = 224Actual Data: SD = 8.79, Mean = 223
"We're multiplying ourSD (8.79) by 3 and then we divide it by 2 to give us the new SD which is 13 (8.79*3/2 = 13). Then we multiply this new SD 13 by 2 to give us the real 2 SD range which is 26. So our range is now 197 - 249. Are we following the right way or not?"
The answer, after the jump...
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