# Guest Essay

## Dietmar Stockl, QC Reality Check, Part Four

**Part three of an ongoing series on Internal Quality Control (IQC) by ****Dietmar Stockl. Dr. Stockl looks again at QC data examples from the real world of the laboratory, this time looking at the case when a laboratory purposes uses the wrong standard deviation.**

## Internal Quality Control (IQC) – A reality check

## Part IV: Purposely working with the “wrong standard deviation”

- Part I: Setting the scene
- Part II: “Pearls on a rope”
- Part III: Purposely working with the “wrong target”
**Part IV: Purposely working with the “wrong standard deviation”**- Part V: How variable/stable do I want it?
- Part VI: How stable can I get it?
- Part VII: What’s going on? – 1

#### DIETMAR STOCKL, PHD

MAY 2011

**Reasoning**

We have several immunoassays that regularly have instabilities that have no clinicical consequences. Therefore, we use the mid-term standard deviation for calculating control limits.

There is significant lot-to-lot variation of several immunoassys which, however, is also not considered clinically relevant. Lot durations are relatively short (1 to 3 months), therefore, we consider the establishment of lot-specific SDs and target values not as cost-effective. We calculate control limits with standard deviations that account for lot variations.

**Lot-example**

Short-term CV (1 lot) = 1.5%: 3s limits, short broken line

Mid-term CV (several lots): 3.1%: 2.58s limits, long broken, line.

**CAVE**

*Stable SD and target value (see Part III) are the heart of QC. If compromises are made at these places, one should be aware of the consequences thereof.*

Do not forget: what you see in your IQC, typically, is reflected in your patient data. If IQC differs, your patients differ! Lot-to-lot variations will affect your patient data!

**QC Problem with purposely (or instinctively) “wrong SD”**

IQC results of an inflammatory protein (µg/mL), measured with a manual ELISA assay.

**Problem description**

“We are violating the 10X rule - which CLIA is concerned about”. “The violation might be because of the same vial being used over time”.

**The chart**

**The data**

The target value 16.83 was established before under stable conditions. The standard deviation of 2.53 was chosen to accommodate “typical instabilities” and lot-to-lot variations; however, the SD of a stable period is 0.9 (day 33 to 68), only!

IQC is done with a 3s rule, and additionally with a 10Xbar rule. There are 2 violations of the 10Xbar rule: in the beginning and in the end.

**Reason for the violation**

The reason is that the “stable” SD (SDstable) is much smaller than the SD used for the definition of the control limits (SDrule). Consequently, even medium-sized shifts/drifts will lead to violations of all rules that work with a mean (average rules) or with a location relative to the target (e.g., the 10Xbar-rule).

**The solution**

If SDstable << SDrule, average rules and X-bar rules are not a good choice for controlling the process.

**Note:** One should have a good justification for these wide limits; in principle, the QC limits are 8.4s limits when SDstable is used for calculation.