Precision Profiles

Illustrates a mixed situation with constant SD in the low range and a proportional relationship in the rest of the analytical measurement range. 

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Figure 6.22 PWG assay dose precision profiles of multiplexed three-analyte immunoassay for two sets of experiments. Dose precisions correspond to standard deviations of analyte concentrations that were back-calculated using corresponding dose response curves. (From Pawlak, M. et al., Proteomics, 2, 383-393, 2002. With permission.)...

The response of an MIP-ILA is nonlinear and the variance is non-uniform so it would be desirable to establish the precision profiles by calculating the standard deviation, or the percentage of the coefficient of variation (%CV) vs concentration. This shows the interval of concentrations where precision is maximum (usually given as 20-80% inhibition). In immunoassays, and by extension in ILAs, this range is considered as the assay working range [45]. 

The precise measurements, then, require the mobility measurements in the cell of precise profile, calculation of the mobility on the stationary level and finally, calculation of the potential, following O Brien and White or Oshima et al. s procedure [87,88]. 

If an assay is being used in which it is customary to duplicate the measurement on each sample within the assay (e.g. immunoassay techniques), then precision profiles can be prepared from the duplicate measurements. This is done by calculating the random error for each pair of samples as a coefficient of variation, given by the expression... 

This group of methods can be subdivided into two subgroups (a) rise or depression inside thin capillaries or pores and (b) that outside objects. The former is the more pronounced feature rises can be substantial when the capillary is narrow and well-wetted. Moreover, it is of great practical relevance. A drawback is that precise profile measurements may be difficult because of optical distortion ). When the capillary is long and thin and the measuring liquid is a surfactant... 

Another reason is that there is a high variation in assay precision close to zero dose, i.e., it usually drops asymptotically parallel with the y-axis reaching a minimum value around the middle of the calibration curve (see Fig. 9.5b). Thus, a considerable difference in CV in the absence of the analyte and at the MDC is expected. This is why the concept of functional sensitivity (FS) was introduced [14], defined as the lowest concentration of analyte for which the CV falls below 20%. FS is a parameter that is far more practical than AS and it is derived from the knowledge of the precision profile (PP), i.e., the changes in precision with the concentration of the analyte (Fig. 9.5b) [14]. 

Fig. 9.5. (a) Bias in estimation of immunoassay sensitivity due to erroneous curve fitting SI, calculated sensitivity S2, true sensitivity, (b) Precision profile (PP) variation of precision (CV in %) for the signal response and estimated concentration (translated from the signal response) within the whole immunoassay range. 

The low detection limits of immunoassays depend on the typically high affinities of antibodies for haptens and antigens, as well as the detection limits of the labels used. The detection limit is usually defined as the concentration that yields a signal that is equal to the mean of the blank signal plus two or three standard deviations. This establishes the confidence range for the zero response. For this calculation, the bound/free versus concentration plot is used. While detection limits allow comparisons of different immunoassay methods at the lower concentration end, they say nothing about assay reliability for this reason, both detection limits and precision profiles should be compared. 

In addition to the usual evaluation parameters for analytical methods (Chapter 16), the sensitivity, detection limit, dynamic range, and precision profile, biosensors are also characterized with respect to the rapidity of their response and recovery. This... 

Precision varies with concentration because of this, precision should be evaluated at the low, middle, and high concentration regions of the standard curve, and should be evaluated in the different matrices that will be encountered in real assays. The precision profile (Section 16.4.3) is used to establish the working range of the assay. 

Precision profiles are useful to establish the concentration range (for a given level of precision) for which a method can be used. Precision is generally worst at the lowest concentration of this range. Figure 16.3 demonstrates how the dynamic (or... 

Precision studies can be performed under different conditions, and are strongly influenced by variables such as temperature, source and quality of reagents, reproducibility of reagent delivery, and instrumental noise. Therefore, if all precision studies are done in the same laboratory (intralaboratory study) higher precision is expected in comparison with interlaboratory studies, where several laboratories produce the data used to prepare the method precision profile. 

Intralaboratory precision studies are classified as intraassay, where the profile is obtained doing the replication in only one run, using the same batch of reagents, or interassay, where the precision profile is obtained by comparison of runs done on different days. Poorer precision is generally obtained in interassay studies. 

The relative uncertainty of measurements at or just exceeding the LoD may be large, and often a quantitative result is not reported. The lower limit for reporting quantitative results, the limit of quantitation (LoQ), relates to the total error being considered acceptable for an assay. From a precision profile for the assay and an evaluation of the bias in the low range, LoQ may be determined in relation to specifications of the method. For example, a laboratory may specify that the total error (e.g., expressed here as Bias -i- 2 SD) of an assay is lower than 45% (corresponding to a bias of 15% and a CV of 15%) of the measurement concentration. " In this case, the LoQ is the lowest assay value at which this specification is fulfilled. LoQ constitutes the lowest limit of the reportable range for quantitative results of an assay. 

Fig. 15.13. Response error relationship (A) and precision profiles (B) to optimize (particularly AM-)EIA according to Ekins (1979). The immunoreactant concentrations which will give minimum variance, will give also minimum variance when small amounts of sample are added and allows the highest detectability. First, the R is measured at different responses (reliability is not high with a few replicates. Section 15.1). The error in dose (AC) or the relative error (AC/C coefficient of variation) can then be determined by the AR (corresponding to the R of that dose) and the sensitivity of the dose-response curve as shown in Fig. 15.1. A change in, e.g., the antibody concentration may change the precision profile from (a) to (b), which has a greater detectability, but less precision at high antigen concentrations.

Figure 1. Precision profile for C-peptide assay Broken lines indicate functional sensitivity (CV=10%) of the assay...

FIGURE 3.8 (a) Precision profile plate map. Four sets of 12 calibrator levels are assayed one predefined set is used to fit a calibration curve, and the remaining sets are treated as samples, (b) Precision profile plot. The %CV of calibrators as samples is plotted versus nominal concentration to give a precision profile. The concentrations at which a certain precision level is met, for example, 15% CV, define the preliminary lower and upper levels of quantification (LLOQ and ULOQ). 

The performance of an assay can be examined through profiling the precision measured for different sample (analyte) concentrations and conditions. Such assessment at any stage can be regarded as a precision profile. 

The precision profile of an assay showing nonuniform error. 

Under the assumption of LTE a rather simple method consists in measuring the dynamic pressure of the gas with a probe. But, in that case, the probe disturbs the plasma flow, even if it is precisely profiled <> >. Another technique consists of the observation of a small electrical perturbation superimposed on the discharge 106) jijg ygg Qf pulsed laser now offers new opportunities for this type of measurement Figure 34 shows the velocity of an argon and nitrogen plasma jet at the nozzle exit as a function of the gas flow rate. A third technique uses very small particles injected into the plasma. In HF plasma the particles velocity, supposed to be the same as the gas flow, is measured by laser anemometry However for DC... 

Pumps cannot develop pressure without imparting some energy or heat. The melt heat increase depends on melt viscosity and the pressure differential between the inlet and the outlet (or AP). The rise can be S F at low viscosity and low AP, and up to 30°F when both these factors are higher. By lowering the melt heat in the extruder, there is practically no heat increase in the pump when AP is low. The result is a more stable process and a higher output rate. This approach can produce precision profiles with a 50 percent closer tolerance and boost output rates 40 percent. Better control of PVC melt heat could increase the output up to 100 percent. In one case, the output of totally unstabilized, clear PVC meat wrap blown film went from 600 to over 1,000 Ib/h with the use of the gear pump. 

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