Dilutions and dilution factors

Parts, volumes, serial steps, and the reported result

Adding diluent lowers concentration. The dilution factor (DF) is final volume divided by specimen volume; DF = 5 gives one fifth of the original concentration.¹

One specimen part in five final parts

Here, 1:5 means specimen:final volume: one specimen part plus four diluent parts. A written 1 + 5 makes six parts, DF = 6. If a method uses specimen:diluent notation, follow its stated volumes.¹

Specimen
100 µLDiluent
100 µLDiluent
100 µLDiluent
100 µLDiluent
100 µL

500 µL final volume = 100 µL specimen + 400 µL diluent
Equal-volume schematic. µL = microliter.

Volumes and factor

Dilution factor: DF = final volume ÷ specimen volume
Specimen volume = final volume ÷ DF
Diluent volume = final − specimen volume

Prepare 1:10, final 1.0 mL.
Specimen = 1.0 ÷ 10 = 0.10 mL.
Diluent = 1.0 − 0.10 = 0.90 mL.
mL = milliliter; 1 mL = 1000 µL.

Concentration and result

Diluted concentration: Original concentration ÷ DF
Original concentration: Diluted concentration × DF

A 1:5 aliquot measures 84 U/L.
84 × 5 = 420 U/L in the original specimen. U/L = units per liter.

The method must recover the expected result after dilution; the choice of diluent can affect that recovery.^1,2

Serial dilutions multiply

Original specimen1:5 step
Total DF = 51:4 step from that mixture
Total DF = 5 × 4 = 20

Each step uses the well-mixed preceding tube. A result of 12 mg/dL in the second tube gives 12 × 20 = 240 mg/dL originally. mg/dL = milligrams per deciliter. Each step dilutes the preceding mixture, so the factors multiply.¹