Clinical Chemistry I 0202304 Lecture Notes PDF

Clinical Chemistry I 0202304 Instructor: Mohammad QABAJAH E-mail: [email protected] Week 03&04 Method Selection and Evaluation 2 Objectives I. To be familiar with the three major areas that aid them into the decision making with regard...

Clinical Chemistry I 0202304 Instructor: Mohammad QABAJAH E-mail: [email protected] Week 03&04 Method Selection and Evaluation 2 Objectives I. To be familiar with the three major areas that aid them into the decision making with regard to analytical method selection II. Understand the analytical performance of analytical methods III. Be aware of both the analytical performance and the practical performance criteria related to automated methods IV. Recall some of the statistical concepts related analytical performance of analytical methods. V. Able to compare between the analytical performance criteria using raw data by Bland-Altman method and correlation regression Method Selection ?? ?? 4 Method Selection: Considerations Medical Usefulness Analytical Performance Practical Criteria Patient Protocol Conditions Method Selection: Medical Usefulness - Achieving optimal patient care - What is needed clinically from a certain lab test Method Selection: Analytical Performance - Calibration - Precision - Accuracy (Trueness) - Analytical Range - Detection Limit - Clinical Sensitivity and Specificity Method Selection: Practical Criteria - The detailed protocol For automated procedures: - Reference materials - Pipetting precision - Reagents composition and stability - Carryover Specimen & Reagent - Technologist skills - Detector imprecision - Possible hazards and waste - Time to report - Specimen requirements - On-board reagent stability - Instrumental requirements - Overall throughput - Cost-effectiveness - Mean time of instrument failures - Computer platforms and interfacing - Mean time to repair - The availability of technical support Statistical Terms - Mean - Standard deviation - Coefficient of variation Gaussian Probability Distribution Analytical Methods: Basic Terms - Calibration - Linearity - Trueness /Accuracy - Precision - Limit of Detection Calibration The correlation between instrument signal and analyte concentration Y = Factor * X Instrument Signal - Y: Instrument Signal - X: Analyte Concentration Analyte Concentration Linearity: Relationship between measured and expected values over the range of analytical measurements Calibrator Known quantity Calibration: - Linear - Curved (immunoassays): Four-parameter-logistic curves Curved function: Nonlinear regression analysis is used or logit transformation Should be monotonic, otherwise, errors occur Stability of the signal &frequency of calibration Calibration - Random dispersion of instrument signal at a given concentration transforms into dispersion on the measurement scale - Modern automated machines: variations in response are very small, this means calibration is stable. Trueness and Accuracy Trueness: Closeness of agreement of the average measured value (high results #) with the True Value Accuracy: Closeness of agreement of a single measurement with True Value Trueness and Accuracy – Related Terms - Recovery: The difference between measured concentration and the amount added - Drift: Differences Caused by instrument or reagent instability over time - Carryover: Differences caused by fraction of a samples measured with the next sample - Bias: mathematical difference between the average and true value  Precision - The closeness between independent results of measurements obtained under stipulated conditions - Imprecision: - Measured by SD or CV - Inversely related to precision - Caused by random error - Types - Between run precession - Interlaboratory pression  Precision – Related Terms - Repeatability: Closeness between results of successive measurements under the same conditions - Reproducibility: Closeness between results of successive measurements under changed conditions - How many measurement: The more observation the more the certainty - Commonly, 20 observations in duplicate Analytical Measurement Range and Limits of Quantification - Measuring Interval Reportable Range - The analyte concentration range over which measurements are within declared tolerances for imprecision and bias of the method Analytical Range Limit of Detection (LoD) & Limit of Blank (LoB) Important for many analytes especially hormones Factors Affecting: Instrument Sensitivity, Background Noise, Sample Matrix, Analyte Properties Errors: Type I error (NO analyte, signal present) Type II error (analyte present, NO signal) Reporting: Not detectable, less than LoD (zero, LoB) Detectable, greater than zero, detected Analytical Sensitivity - The ability of an analytical method to assess small variations in the concentration of analyte (Smallest concentration or amount of an analyte that can be measured and quantified with a high degree of accuracy and precision) - Depends on - Slope of the calibration curve (direct relationship) - Random variation (precision, CV) (inverse relationship) Analytical Specificity and Interference The ability of an assay procedure to determine specifically the concentration of the target analyte in the presence of potentially interfering substances or factors in the sample matrix Analytical Goals Based on: Clinical outcomes of clinical setting Biological variation Imprecision (σT)2 = (σ)2 within-B+ (σ)2A bias < 0.25 (σ2 within-B+ σ2between-B)0.5 Limits set by regulatory bodies Method Comparison Comparison study (points to be considered) - The number of samples necessary - The distribution of analyte concentrations - The representativeness of the samples - Practical aspects: Storage; treatment of samples; anticoagulants; measurement times - Ethical issues Comparison of Methods_Difference Bland-Altman Plot X1 X2 (X1 + X2)/2 X2-X1 (X2-X1)/((X2-X1)/2) 0.01005 99 100 99.5 1 190 200 195 10 0.051282 -0.03922 260 250 255 -10 Comparison of Methods_Regression Analysis - Estimate relationships between a dependent variable and one or more independent variables - Ordinary Least-Squares [OLS] - Beware of the outlier - https://www.youtube.com/watch?v=P8hT5nDai6A X2 X1 100 99 200 190 250 260 Comparison of Methods_Proportional random error - Depends on the change in a specific variable (directly related) - Measurable amount: X divided by Y always equals the same constant. Comparison of Methods_Traceability Unbroken chain of comparisons of measurements leading to a known reference value the property of the result of a measurement or the value of a standard whereby it can be related to stated reference through unbroken chain of comparisons (chain of calibration leading to primary national or international standards) Comparison of Methods_Uncertainty A parameter associated with the result of a measurement that characterizes the dispersion of the values. Expressed by SD The location of true value with a given probability ~ 95 % Affected by - Preanalytical variation - Method imprecision - Sample-related random interferences - Uncertainty related to calibration - Bias corrections (traceability) See YOU Next Lecture ☺