Dilution Math

A manual dilution has two calculations: divide the finished mixture into equal parts, then apply the factor that matches the mixture actually made.

Parts and dilution factor

A dilution ratio counts parts of the finished mixture. In a 1:5 dilution, the specimen is one part and the finished mixture is five parts. Four parts are diluent, and the dilution factor is 5.

In clinical chemistry, a 1:5 dilution contains one specimen part in five total parts. A specimen-to-diluent ratio describes the components separately; its stated volumes and correction factor govern preparation.¹

Written as 1 + 5, the notation means one part specimen plus five parts diluent. That is six finished parts with a factor of 6.

For a chosen final volume:

specimen volume = final volume ÷ total parts

diluent volume = final volume − specimen volume

The Dilution Bench applies these relationships while a 1:5 dilution is prepared, measured, and corrected.

Concentration and correction

The diluted concentration depends on the fraction of the finished mixture occupied by specimen:

diluted concentration = original concentration × specimen volume ÷ finished volume

This form also describes a mixture that misses its intended final volume. The factor used to correct the result must match the mixture actually made.

corrected result = displayed diluted result × dilution factor

Serial dilution factors multiply. Two successive fivefold dilutions have a total factor of 25.

Measuring intervals

The analytical measurement range is the interval a method measures directly without dilution, concentration, or another pretreatment outside its usual assay process. Reportable range may also refer more broadly to values made reportable through dilution.²

CLSI calls the direct interval the analytical measuring interval and the interval extended by an established dilution scheme the extended measuring interval. The diluted aliquot must remain inside the interval supported by that scheme.³

A manufacturer may establish the dilution scheme and its claims, which the laboratory then verifies. When those claims are inadequate, the laboratory may establish and verify a local scheme. The scheme specifies the diluent and permitted ratios. Precision across the interval is part of establishing or verifying the scheme, particularly at high dilution ratios. ³

Reporting requirements

The Dilution Bench begins with an initial CK result above its 50–1000 U/L direct measuring interval. Its analyzer displays one decimal place before the dilution factor is applied. The correct 100 µL specimen and 400 µL diluent preparation reads 836.0 U/L and produces a corrected result of 4180.0 U/L with factor 5. Adding 500 µL diluent makes six total parts; applying factor 5 to its 696.7 U/L reading produces 3483.5 U/L. The assay instructions and laboratory procedure specify the diluent, permitted ratios, measuring interval, significant digits, rounding, dilution record, and reporting limits used for patient testing.

References

1. Bishop ML, Fody EP, Van Siclen C, Mistler JM, Moy M. Clinical Chemistry: Principles, Techniques, and Correlations. 9th ed. Jones & Bartlett Learning; 2023.
2. College of American Pathologists. Analytical Measurement Range. Accessed August 25, 2026. Applies to CAP-accredited laboratories.
3. Clinical and Laboratory Standards Institute. Establishing and Verifying an Extended Measuring Interval Through Specimen Dilution and Spiking. 1st ed. CLSI guideline EP34. Clinical and Laboratory Standards Institute; 2018. Reaffirmed March 2023. Accessed August 25, 2026.