Posted by Sten Westgard, MS
An interesting question came in through email (
"1. Can we apply Westgard multirules to hematology control?
2. I read in one paper that the hematology control range is calculated as mean +/- 2.5 SD, is this correct and if not how can I calculate own laboratory hematology control range?"
The answer, after the jump...
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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 and
where can I find more information about what is
meant by those terms? I was not able to find this
information, but laboratory leadership staff said
that "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 run
or back-to-back runs".
A lab has the following multi-rules; "Within QC"
1:3s, 1 QC result outside 3sd; 2:2s, 2 consecutive
QC results outside 2sd on the same side of the mean;
4s, 2 consecutive Qc results differ by more than
4sd; and 1:2s, 1 Qc result outside 2sd and within 3.
(1:2s is used as a warning rule, the others as
rejection rules). The rules for "Across Qc" are as
follows; 2:2s, 2 consecutive Qc results (1 each
multiple levels) are outside of 2sd; and 4s, 2 Qc
results (one of each multiple levels) are >4sd
apart. These are both rejection rules. These
multi-rules are used to assess all tests in a
chemistry lab; the majority of tests are assessed
with 2 levels of Qc, a few use 3 levels of Qc.
The situation arose where QC results on one day for
a cancer antigen were the following:
Day 1A:
Level 1 -Within 2sd, acceptable
Level 2 -1:2s, run was accepted as only the warning
rule 1:2s was encountered.
The next day the results were as follows:
Day 2A:
Level 1 -1:2s
Level 2 -Within 2sd
Leadership said run should not be accepted,
violating the "across" 2:2s rule.
However, leadership said the inverse situation would
have been acceptable as *consecutive* data points
did not violate the "across" 2:2s rule, i.e.
Day 1B:
Level 1 1:2s
Level 2 within 2sd
Day 2B:
Level 1 within 2sd
Level 2 1:2s
In the A group, because Level 2 is outside of 2s,
and the very next data point (Level 1 from the next
day) is also out 2s, the run is unacceptable and
should be rejected. In Group B, since consecutive
data points are okay the run is acceptable.
Is this a correct approach? Is it correct to reject
group A (Day 1A and 2A) and not reject group B (Day
1B and 2B)? Do these multi-rules as outlined and
implemented detect some unacceptable variation in
group A that does not exist in group B? Thank you
for 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 = 224
Actual 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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Here's a question from a website visitor regarding assigning a mean value for a new QC material with the following assumptions:
"1. The analyte reports out as a whole number.
2. The results of calculations on 20 replicate samples are;
A. Mean = 10.5
B. SD = 0.5
C. 2 SD Range = 9.5 - 11.5
D. 95% Confidence Interval = 10.3 - 10.7
E. CV% = 4.9
The question is "what to set the mean at?" One camp contends that the mean of 10.5 should be used, even though no result will ever "hit" the mean. The other camp states that the mean should be set to 10 or 11 regardless of whether or not a LJ shows bias, or even 10x failure. "
Answer after the fold.
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Posted by Sten Westgard, MS
This question comes to us from a CLS student in Texas:
"I was hoping that someone might be able to answer a question that is causing me and some of my
classmates some confusion. There is some confusion when you are plotting your QC chart and all your values are a whole number. Would you keep you SD as a whole number or make it to one decimal place? And if you did make it to one decimal place, would you be making your SD more accurate that your original value?"
Answer after the jump...
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Posted by Sten Westgard, MS
We want your control chart data!
We've always had an "open door" policy on the website, welcoming your questions and your data. At times, we may not be so explicit in inviting you to contact us with your observations, comments, inquiries, and frustrations, but we do want to hear from you.
Now, we're asking for a more specific set of data from you - for your problem methods, show us a few months of QC data, ideally with notations such as new control lot, new reagent, recalibration, etc.
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Posted by Sten Westgard, MS
Posted by Sten Westgard, MS
A recent question came in from outside the US, regarding the choice of controls and EQA/PT provider. In such a case, there are often economies gained by choosing the same vendor for both control materials and EQA/PT materials. But the theory of quality control and assurance dictates that you should have independent checks on your processes. Indeed, in some countries, there are regulations which prevent control manufacturers from selling proficiency testing materials for precisely that reason.
This is a difficult issue, one that epitomizes the struggle between efficiency (cost) and safety (quality).
More after the fold...
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Posted by Sten Westgard, MS
Posted by Sten Westgard, MS
Posted by Sten Westgard, MS
Often we get questions about the problems that laboratories encounter when they wrestle with their local regulations.
Here we have a case where there are Turkish laboratory regulations that are irrational. In the words of the submitter:
"I have a question about the estimation of total CV. We have a requlation about total allowable error in Turkey....It is Turkish, but I strongly believe that you can follow it.
"The marked page can be seen below: There is an example for glucose. Two levels of controls are measured and the CV%’s calculated. The total CV are calculated from the CV’s of two different levels as seen in the equation. I can not accept it, because the levels are different even if the CV’s are close."
What's gone Rong here? More after the jump...
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Posted by Sten Westgard, MS
"We have a department supervisor that instructs the techs to delete (not omit) qc values outside of three s.d..Is there a specific CLIA rule against this? Where I come from, deleting qc values is wrong."
What do you think? The answer, after the jump...
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Posted by Sten Westgard, MS
Another question coming in from one of our website members:
Could you give me some suggestions on establishing QC ranges for unassayed chem. Controls?
We currently use [Brand X] unassayed chem controls (much less in cost than assayed controls) for some of our chemistry analytes.
For these controls, we are provided a “target mean” and a range.
Occasionally, our established mean (i.e. n=30) for a new lot is outside of the range provided for “target mean” provided by manufacturer.
Could you suggest guidelines for acceptance of lab established means for unassayed controls?
Obviously, I would like to know how far from the “target mean” could the new lab established mean be?
The answer, following the jump...
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Posted by Sten Westgard, MS
We got the following question emailed to the website recently:
Greetings, I was [wondering] if the following case represents [violation] of 4 1s rule?
Control (A) point 1= EXCEEDS 2 SD /-2 SD lines >> warning
Control (A) point 2= WITHIN 2 SD/-2 SD lines >> accepted
Control (A) point 3= EXCEEDS 2 SD /-2 SD lines >> warning
Control (A) point 4= WITHIN 2 SD/-2 SD lines >> rejection
all the point[s] on the same side of the mean.
What do you think? The answer, after the jump
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Posted by Sten Westgard, MS
I had the pleasure of taking part (albeit remotely) in the Quality at the Crossroads conference in Alexandria, Egypt.
As part of my session, we took questions from the audience, and I thought I would share a few of them with you, as well as a few of the "wrinkles" that labs in Egypt experience that we in the US do not. So here are the questions:
- How frequently should I measure Total Error and Measurement Uncertainty?
- Can I resort to comparing my EQA result to allowable bias when EQA result is violated and declared incorrect due to tight SD of comparator group?
- When using a new manufacturer’s QC, we should establish our own mean and CV, but it would result in tight CV and shift after 20 days. What are the recommendations for this? Is it better to wait more days to establish CV?
Some answers, after the jump...
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Posted by Sten Westgard, MS
An interesting question came from one of our readers, about how to interpret certain outliers.
So let's take one example, just two control values, expressed in z-scores. If the low control is -3.1 and the high control is + 2.3, which rule is violated? the 1:3s rule? The R:4s rule? Both?
An answer, after the jump...
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Posted by Sten Westgard, MS
A recent question came in from a technical support consultant for a major diagnostic manufacturer:
"It is for Free T4 analyte. Customer [has] establihed a QC range after 20 QC runs. Mean and SD were derived from 20 runs and %CV achieved from 20 runs is 1.2%. Allowable interassay precision criteria according to CLIA is 6%.
"Now when customer [applies] Mean and SD according to the established range many...times they face QC rules violations of 41s and 10x.
"So can you please suggest, when the SD is too narrow is it necessary to apply these two rules to immunoassays?"
What's your guess? I'll take a stab at an answer, after the jump.
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