Serial dilutions? Don’t! [Study As­sess­ment]

posted by Helmut Homepage – Vienna, Austria, 2017-10-30 17:53  – Posting: # 17955
Views: 6,195

Hi öberster größter Meister,

» dilute the sample you already diluted; use the already validated dilution factor and inject. Continue doing that until you get a measurable concentration.

Really? Quoting the BMV-GL:

4.1.7. Dilution integrity
Dilution of samples should not affect the accuracy and precision. If applicable, dilution integrity should be demonstrated by spiking the matrix with an analyte concentration above the ULOQ and diluting this sample with blank matrix (at least five determinations per dilution factor). Accuracy and precision should be within the set criteria, i.e. within ±15%. Dilution integrity should cover the dilution applied to the study samples.

(my emphasis)

If you want to dilute ×2, ×4, ×8 those are the dilution factors – not serial ones like 1×2, 2x2, 2x2x2. Imagine a propagation of error which results in an accuracy of 90% in each of the steps. If done properly (only one dilution), you pass the requirements (within ±15%). With serial dilutions you would fail already in the second step (81% outside ±15%) and even more in the third (73%). But: Since you didn’t validate it, you falsely assume that the accuracy is still 90%. You don’t want to go there.
Of course, you could validate serial dilutions as well but I don’t see any rationale for doing so.

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Helmut Schütz

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