**Method validation**

**Method performance characteristics and related acceptance criteria**

**- linearity**(measurement area);

**- measurement repeatability**- precision under the repeatability condition of measurement – same analyst, same sample, same measuring system, same operating conditions, same location, short period of time (frequently used term for this concept in clinical chemistry and laboratory medicine is intra-assay precision);

**- intermediate precision**– precision which is achieved within the same laboratory over an extended period of time but may include other conditions involving changes: new calibrations, calibrators, operators, and measuring systems;

**- measurement reproducibility**- precision under the reproducibility condition - precision which is reached between laboratories (it is usually not quantified in case of in-house method validation, but it is an important parameter in method standardization);

**- measurement trueness:**

*b*) and/or recovery;

**- selectivity**;

**- limits of detection**;

**- limits of quantification**;

**- method robustness**.

**Target measurement uncertainty and acceptance criteria**

**Measurement uncertainty**

*Guide to the Expression of Uncertainty in Measurement*(GUM) (2) issued in 1993, corrected in 1995. The following international organizations participated in its assembly: Bureau international des poids et mesures (BIPM), International Electrotechnical Commission (IEC)

**,**International Federation of Clinical Chemistry (IFCC), International Organization for Standardization (ISO), International Union of Pure and Applied Chemistry (IUPAC), International Union for Pure and Applied Physics (IUPAP) and International Organization of Legal Metrology (OIML).

**GUM describes two ways of evaluation – type A, estimated by statistical means, and type B, estimated by other means.**

*k*which ensures the agreed coverage probability, usually P = 95 %.

**How to include data from validation experiments into measurement uncertainty estimation?**

*s*) or coefficients of variation (CV). Out of the three precision levels mentioned (repeatability, intermediate precision and reproducibility), the most interesting one in measurement uncertainty assessment (made from validation experiments) is intermediate precision since it includes much wider sources of random errors than it would be the case with repeatability. Reproducibility is not established in an in-house validation.

*b*), is investigated by comparing the expected reference value (x

_{ref}) with the estimation of the result given by the method (x):

*b*= x̄

*– x*

_{ref}

**Case 1: uncertainty when correction is applied**

_{kor}) is reported as:

_{kor}= y – b.

*b*) should be sufficiently accurate, well established, and significant in size. Then only the uncertainty of correction

*u*

_{b }enters into the calculation of uncertainty:

*x*

_{ref}).

*n*is the number of observations made in this trueness experiment.

*u*(

*x*

_{ref}) is measurement uncertainty associated with the quantity value of a reference material (type B evaluation GUM (2)).

*s*), then the measurement uncertainty of the corrected result is:

*s*is standard deviation obtained from intermediate precision experiment.

*u*

_{b}is a component of uncertainty due to estimation of bias.

*Y*=

*y*

_{kor}_{ }±

*ku*(

*y*).

_{kor}*k*is equal to 2.

**2.**

**Case: Uncertainty when correction is not applied**

*b*), in some cases it might not be practical and feasible or might be too expensive. The correction of results may require modifications to existing software and “paper and pencil” corrections can be time consuming and prone to error. In such very special circumstances when a known correction

*b*for systematic effect cannot be applied, the “uncertainty” assigned to the result can be enlarged by it. Several methods could be applied for this. GUM (1) describes a method when such enlarged “uncertainty” is the sum of expanded uncertainty and a known systematic effect (

*b*). The measurement result is reported as:

*Y*=

*y*± (

*ku*(

*y*) + I

_{kor}*b*I).

*b*) is treated as an uncertainty component:

*b*>>

*s*.