Clinical Chemistry Lab - 02 Quality Control and Statistics

CLINICAL CHEMISTRY LABORATORY • QUALITY CONTROL: STATISTICS & QC CHARTS OUTLINE • • • Statistics o Descriptive Statistics o Inferential Statistics QC Charts o Gaussian Curve o Youden/Twin Plot o Shewhart Levey-Jennings Chart o Westgard Chart o Interpretation of QC Result Terminologies Standard Deviation (SD) o Measure of dispersion of values from the mean. o Measure of distribution range. o It helps describe the normal curve. o Best measure of random error (indeterminate error) o Ideal: 2SD or lower ▪ Reject when above the ideal o Measures precision ▪ Inversely related to SD ▪ ↑SD = dispersed values = ↓ precision ▪ ↓SD = values close to each other = ↑ precision o Formula STATISTICS • Quality control data are analyzed by statistic and presented through QC Charts o DESCRIPTIVE STATISTICS • • • • Used for monitoring test performance and quality control o Commonly used in the laboratory Assessment of data dispersion allows laboratorians to assess predictability in a laboratory test or measurement Involves: o Measures of center • Mean - average • Median – middle data after the data have been ranked order • Mode – the most frequently occurring data o Measure of dispersion ▪ Standard deviation ▪ Coefficient of Variation ▪ Variance Mean (x̄) o aka Average o Measure of central tendency o Summation of X (σ x) o The only measure of accuracy o Formula o • n-1 = degree offreedom Sample problem: What is the standard deviation of the control values of glucose obtained after 5 consecutive days? Coefficient of Variation (CV) o Percentile expression of the mean o Index or 2nd measure of Precision o Ideal: 2 to 4% o Formula o Sample problem: What is the coefficient of variation of the control values of glucose obtained after 5 consecutive days if: Sample problem: What is the mean of the control values of glucose obtained after 5 consecutive days? • Variance (V) o aka standard deviation squared o Square of dispersion o Measure of variability o Represents difference between each value and mean o Formula: V = (SD)2 o Sample problem: What is the variance of the control values of glucose obtained after 5 consecutive days if: SD = 2.65 V = (SD)2 V = (2.65)2 V = 7.02 INFERENTIAL STATISTICS • • • • • Used to compare means or SD of two groups of data Used to determine significant difference between two groups of data** Commonly used in experimental and quantitative research T-test o used to determine whether there is a statistically significant difference between the means of two groups of data F-test o used to determine whether there is a statistically significant difference between the SD of two groups of data YOUDEN/TWIN PLOT QC CHARTS • • Used to observe control values over time to determine reliability of the analytical method. Utilized to observe and detect analytic error such as inaccuracy and imprecision. GAUSSIAN CURVE • • • • • • • Aka Bell-Shaped Curve, Normal Frequency/Distribution Curve Obtained by plotting the values from multiple analyses of a sample. Bell-shaped curve – occurs when data elements are centered around the mean with most elements close to the mean 1.0/100% - total area under the curve Used to compare results obtained on a high and low control serum from different laboratories. Displays results: o Y-axis – mean values of one specimen o X-axis – mean values of another specimen Detects systematic error: o Proportional Error – points fall on the center but on the 45 ̊ line. o Constant Error – points fall on the center but NOT on the 45 ̊ line SHEWHART LEVEY-JENNINGS CHART • • • • • • ** Values from -2SD to -3SD & 2SD to 3SD are invalid CUMULATIVE SUM GRAPH (CUSUM) • • • • • Calculate the difference between QC results and the target means Common Method: V-mask method Identifies the earliest indication of systematic problem Very sensitive to small, persistent errors. >45 ̊ slope (>2.7 SD) – out-of-control! aka Dot Chart A graphical representation of the acceptable limits of variations in the results on an analytical method. Only control values are plotted in LJ Chart o Most widely used QC chart in the clinical laboratory. Best Internal QC chart o Detects both random and systematic errors o Allows laboratorians to apply multiple rules without computer aid ERRORS OBSERVED IN LJ CHART • Trend o Gradual loss of reliability in the test system o Seen visually in LJ Chart ▪ at least 6 consecutive increase or decrease in control values o Cause: ▪ Deterioration of reagent ▪ Deterioration of light source/control materials. WESTGARD CHART • • • • aka Multiple Rule Chart States that the use of simple upper and lower control limits are not enough to identify analytical problems Error detection can increase without increasing the false rejection rate. Control rule – indicates if the analytical process is out-ofcontrol. WESTGARD CONTROL RULES • • Shift o Shift in reference range is due to transient instrument differences o at least 6 consecutive control values situated on either side of the mean. ▪ Neither decreasing or increasing ▪ Do not cross the mean line o Cause: ▪ Improper Calibration of machine ▪ Change in reagent formulation • 12s o One control value exceeds the mean +/- 2SD. o Warning Rule (for screening purposes) ▪ Only rule that do not require repeat • 13s o One control value exceeds the mean +/- 3SD. o Determines random errors o If occurs, repeat quality control • 22s o Two consecutive control values exceed the mean +/2SD. o Determines systematic errors • 41s o Four consecutive control values exceed mean +/1SD. o Determines systematic errors Outlier o Control value is far from the main set of values o Highly deviating value (≥3 +/- SD) o Caused by random or systematic error in analytical method • • • R4s o The range or difference between two control values exceed 4SD. o Determines random errors o Similar to LJ Chart Outlier 6x o o o 7T o o • 8x o o • 10x o 10 consecutive control values fall on one side of mean. o Do not cross the mean line • 12x o 12 consecutive control values fall on one side of mean. o Do not cross the mean line • 31s o 3 consecutive control values exceed mean +/- 1SD. • 2 of 32s o If 2 out of 3 consecutive control values exceed mean +/- 2SD • Reject control run if it meets any of westgard rule criteria (except 12s) Multirule Shewart (LJ Chart) – control chart + control rules 6 consecutive control values fall on one side of mean. Do not cross the mean line Similar to LJ Chart Shift Seven control values trend in the same direction (progressively increasing or progressively decreasing). Similar to LJ Chart Trend 8 consecutive control values fall on one side of mean. Do not cross the mean line • • 9x o o 9 consecutive control values fall on one side of mean. Do not cross the mean line INTERPRETATION OF QC RESULT • • • • +/- 2SD – acceptable reference limit If control value exceed +/- 2SD – run new set of control and repeat specimen testing** If control value is between +/- 2SD and +/-3SD – sign of potential problem If control value is outside +/- 3SD – perform corrective action o If the problem is already identified – retest new set of control material • Continuous QC failures requires more troubleshooting: o Preparation of new reagents o Recalibration o Instrument maintenance and repair TERMINOLOGIES • • • Delta Check o Patient-based quality control technique o The difference between two consecutive measurements of the same analytes on the same individual o It requires computerization of test data so that current results can be compared with past results. Analytical Run o A set of control and patient specimens assayed, evaluated, and reported together. Reference Value o aka Reference Limit, Reference Interval o previously termed as “Normal value” o Range of values into which 95% of non-diseased individuals will fall o The usual values of a healthy population that represents 95% central tendency. • Step 3: Calculate the upper and lower control limits (±1SD to ±3SD) • Step 4: Form your chart o X-axis – day 1 to day x ▪ number of control values o Y-axis ▪ Upper Control Limits • +1SD, +2SD, +3SD ▪ Mean ▪ Lower Control Limits • -1SD, -2SD, - 3SD • Step 5: Plot your values inside the chart. o Make sure that the values are within the range. • Step 6: Analyze your chart using Westgard Multirule System. o To detect random and systematic errors. o To detect accuracy and precision. HOW TO CREATE A SHEWART LEVEY-JENNINGS CHART • Create a Shewhart Levey-Jennings Chart for the following control values of creatinine performed for 15 consecutive days. • Step 1: Tabulate the Control Values o Tabulation of data must be in order of analyses (day 1 to day x) • Step 2: Calculate for the Mean and SD. o You may compute using scientific calculator or using EXCEL.