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Assurance Indicators

• Number of patient redraws

• Labeling errors

• Patient and specimens properly identified

• Critical values called

• Pass rate on competency testing

• Test cancellation

• Integrity of send-out samples

• Employee productivity

• Errors in data entry

• Testing turnaround times

• Delays due to equipment failures or maintenance

• Performance on proficiency testing

• Number of patient redraws

• Labeling errors

• Patient and specimens properly identified

• Critical values called

• Pass rate on competency testing

• Test cancellation

• Integrity of send-out samples

• Employee productivity

• Errors in data entry

• Testing turnaround times

• Delays due to equipment failures or maintenance

• Performance on proficiency testing

Quality control is a large part of the quality assurance program at most facilities. Students will be introduced to the term quality control early and often. It is an essential function in the clinical laboratory. The information that follows provides a brief overview of the quality control procedures used in promoting quality assurance in the hematology laboratory. It is not intended to be comprehensive but introduces terminology and concepts pertinent to the entry-level professional.

Quality Control Monitoring in the Hematology Laboratory

The analytical component, or the actual measurement of the analyte in body fluids, is monitored in the laboratory by quality control, a component of the laboratory quality assurance plan. Similar to the chemistry laboratory, the analytic method in the hema-tology laboratory primarily includes instrumentation and reagents. Standards, or calibrators, are solutions that have a known amount of an analyte and are used to calibrate the method. A standard, or calibrator, has one assigned, or fixed, value.7 For example, the hemoglobin standard is 12 g/100 mL, meaning that there is exactly 12 g of hemoglobin in 100 mL of solution. Conversely, controls, or control materials, are used to monitor the performance of a method after calibration. Control materials are assayed concurrently with patient samples, and the analyte value for the controls is calculated from the calibration data in the same manner as the unknown or patient's results are calculated.7

Control materials are commercially available as stable or liquid materials that are analyzed concurrently with the unknown samples. The control material measured values are compared with their expected values or target range. Acceptance or rejection of the unknown (patient) sample results is dependent on this evaluation process.

A statistical quality control system is used to establish the target range. The procedure involves obtaining at least 20 control values for the analyte to be measured. Ideally, the repeated control results should be the same; however, there will always be variability in the assay. The concept of clustering of the data points about one value is known as central tendency. The mean, mode, and median are statistical parameters used to measure the central tendency. The mean is the arithmetic average of a group of data points; the mode is the value occurring most frequently; and the median is the middle value of a dataset. If the mean, mode, and the median are nearly the same for the control values, the data have a normal distribution.7

The standard deviation and coefficient of variation are a measure of the spread of the data within the distribution about the mean. Standard deviation is a precision measurement that describes the average "distance" of each data point from the mean in a normal distribution. This measurement is mathematically calculated for a group of numbers. If the measured control values follow a normal distribution curve, 68.6% of the measured values fall within the mean and one standard deviation (SD) from the mean, 95.5% falls within the mean and two standard deviations (2SD) from the mean, and 99.7% fall within the mean and three standard deviations (3SD) from the mean. The 95.5% confidence interval is the accepted limit for the clinical laboratory.

Coefficient of variation (CV) is the standard deviation expressed as a percentage. The lower the CV, the more precise are the data. The usual CV for laboratory results is less than 5%, which indicated that the distribution is tighter around the mean value.8

Clarifying accuracy and precision is usually a troublesome task as these terms are often used interchangeably. When a test result is accurate, it means that it has come closest to the correct value if the reference or correct value is known. In most cases, once a methodology has been established for a particular analysis, standard or reference material is run to establish a reference interval. Accuracy is defined as the best estimate of the result to the true value.9

Precision relates to reproducibility and repeatability of test samples using the same methodology. Theoretically, patient results should be repeatable if analyzed a number of times using the same method. If there is great variability of results around a target value, then the precision is compromised8 (Fig. 1.6).

Normal, or Reference, Intervals

Normal, or reference, intervals are values that have been established for a particular analyte, method, or instrument and a particular patient population. To establish a reference interval, the size of the sample must be at least 25 and should represent healthy male and female adults as well as the pediatric population. Once the test samples have been analyzed under predetermined conditions, a set reference value is determined from which reference limits and reference intervals may be established according to statistical methods. Subsequent patient samples will be compared to the reference interval to determine if they are normal or outside of the reference interval.10

10 PartI • Basic Hematology Principles

10 PartI • Basic Hematology Principles

Figure 1.6 Is it accurate or precise? (A) Shots are neither accurate nor precise. (B) Shots are precise but miss the mark, not accurate. (C) Shots are accurate and precise.

Delta Checks

Delta checks are a function of the laboratory information system. This function allows the operator to perform a historical check on the sample from the previous results. If the variation in patient sample exceeds the established standard set for delta checks, a cause must be identified. Preanalytic problems, misidentified samples, analytical errors, or changes in the patient condition may contribute to erratic delta checks.

Reflex Testing

If automated complete blood count (CBC) results present a flagging signal, operations must be performed by the technologist to validate this sample. Usually flags are displayed next to a specific result. For example, an "H" indicates high results, while an "L" indicates low results. However, multiple flags may be generated for the entire CBC. Manual methods may be needed, or additional tests (e.g., adding a differential count or manual slide review) may need to be performed on the sample to present accurate test results. Technologists should be vigilant when hematological data are flagged, because it almost always means that the sample has some abnormality.

Preanalytic Variables

Preanalytic variables refer to any factors that may affect the sample before testing. Some issues to be considered are whether the sample was properly identified, properly collected in the correct anticoagulant, and delivered to the testing facility in a timely fashion. See Table 1.3 for a list of preanalytic variables.

Postanalytic Variables

This term refers to operations that ensure the integrity of sample results. Some examples are proper documentation of test results, timely reporting of results to a designated individual if a critical result was observed, and proper handling of samples that may involve calculations or dilutions. See Table 1.4 for postanalytic variables.

Table 1.3 O Preanalytic Variables

• Proper patient identification

• Properly labeled tubes

• Proper anticoagulant

• Proper mixing of sample

• Timely delivery to laboratory

• Tubes checked for clots

• Medications administered to the patient

• Previous blood transfusions

• Intravenous line contamination

• Blood sample properly collected (proper tube, proper anticoagulant)

Critical Results

Critical results are those results that exceed or are markedly decreased from the reference range or the patient's history of results. These results are usually flagged by the automated instrument. It is essential that either the physician or the appropriate designee be notified immediately by a member of the reporting laboratory, as many critical results involve immediate medical or patient care decisions.

Table 1.4 O Postanalytic Variables

Delta checks Results released Critical results called Reflex testing initiated Specimen check for clots

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