What can a dilution check tell me about the reportable range?

Separate the measuring interval from the extension

Suppose a fictional method has an analytical measuring interval of 2–100 units for specimens measured without an extra dilution. A specimen gives a result above 100 units. The laboratory wants to know whether a specified dilution can bring the measured portion within the interval and still give a reliable estimate of the concentration in the original specimen. An extended measuring interval depends on evidence about the procedure used for that extra step. [1]

Multiplying the upper limit by a proposed dilution factor gives a theoretical maximum. In this example, a twofold dilution and a 100-unit upper limit give 200 units. The calculation does not establish the upper end of a usable extended interval. That depends on the applicable instructions, the specimen matrix, the dilution procedure and evidence at the concentrations concerned. [1]

Follow one specimen through two dilutions

In this illustration the same specimen has a value of 160 units assigned independently using a suitable comparison procedure. The laboratory prepares two dilutions with the specified diluent. The twofold dilution contains one part specimen and one part diluent. The fourfold dilution contains one part specimen and three parts diluent. Both diluted readings fall inside 2–100 units.

Check recovery as well as the instrument reading

The laboratory has set a fictional recovery criterion of 90–110% for the example. The result at twofold dilution meets this criterion for this specimen. The result at fourfold dilution is below it, although the diluted reading of 33 units is comfortably within the analytical measuring interval.

The difference could reflect the preparation, the interaction between the specimen and diluent or another part of the measurement procedure. One comparison establishes the discrepancy in this example and does not identify the cause. The independently assigned value also matters. If the expected value comes only from another unverified dilution of the same specimen, the comparison tests consistency between dilutions rather than recovery against an independent value. [1]

Define the scope of what the check supports

The result at twofold dilution gives one piece of evidence at about 160 units under the stated conditions. An extended interval needs evidence across the concentrations and dilution factors the laboratory intends to use. It also needs an appropriate decision criterion and a clearly specified procedure. A linearity study within the undiluted interval addresses a different question from recovery after dilution above it. [1] [2]

Example wordingAt 160 units, the twofold dilution gave 97.5% recovery under the conditions tested. The fourfold dilution gave 82.5%, below the illustrative criterion. This single specimen provides evidence at one concentration and does not establish the upper limit of an extended measuring interval.