All you wanted to know aboutlaboratory tests, but were afraid to

ask

By Dietmar StöcklSTT Consulting

- 5 -

The informed patient meeting the laboratory director

MetrologyAccuracy and more 2

All you wanted to know about laboratory tests -5- 2

Accuracy and more

TodayWhat would Westgard say?

Sounds good!

All you wanted to know about laboratory tests -5- 3

Repetition

Trueness, NOTE 2 Measurement trueness is inversely related to systematic measurement error.

Bias [of measurement]Estimate of a systematic measurement error.

Random error, NOTE 2 Random measurement errors of a set of replicate measurements form a distribution that can be summarized by its expectation, which is generally assumed to be zero, and its variance.

All you wanted to know about laboratory tests -5- 4

What would Westgard say?

JIM! How does that sound to you?

Similar, but not the same!

All you wanted to know about laboratory tests -5- 5

How do they compare?

VIMTrueness, NOTE 2 … trueness is inversely related to systematic … error.

Bias [of measurement]Estimate of a systematic … error.

Random error, NOTE 2 Random … errors of a set of replicate measurements form a distribution that can be summarized by its … variance.

All you wanted to know about laboratory tests -5- 6

How do they compare?VIM

Bias [of measurement]

Random error, NOTE 2 Note 2: "Distribution-based", probability not specifieds2 (variance)AND "result-based" (definition)

Error"Result-based"Measured value minus a reference value.

Westgard

Bias

Random error"Distribution-based", with specified

probability (95%: two-, or one-sided)z x s (z = 1.96 or 1.65)

Total error"Distribution-based"

Bias + z x s

All you wanted to know about laboratory tests -5- 7

How do they compare?VIM

AccuracyCloseness of agreement between a measured quantity value and a true quantity value of the measurand.

Westgard

AccuracyLike it or not, manufacturers and

laboratories are left with the “systematic error concept of accuracy”

(westgard.com/essay118.htm)

Still differences!

All you wanted to know about laboratory tests -5- 8

Another voice

Typical total error concepts are too simplistic.

At least, sample related effects should be included.

Krouwer JS.

Setting performance goals and

evaluating total analytical error

for diagnostic assays.

Clin Chem 2002;48:919-27.

All you wanted to know about laboratory tests -5- 9

Errors – SummaryRandom error

-Analytical: SDa (CVa)

-Sample-related: SDs (CVs)

“Total”: SDt = [SDa2 + SDs

2] (may be expanded)

Systematic error/Bias

Bt = Experimental mean (sufficient n) minus “reference”/”true”

Overall plus actual lot (may be expanded)

Total error

TE = Bt + z SDt

Interferences

Lipaemia, heterophilic antibodies, etc.

>Estimate frequency and magnitude of effect

>Treat separately by risk management procedures (my opinion)

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In the end

I type: ... altogether, there still is hope ...

In my lifetime?

All you wanted to know about laboratory tests -5- 11

The next visit

Uncertainty

I love that one!