9: Quality Control

Mean

Most commonly used and often referred to as the average

Median

Middle point of the data and is often used with skewed data

Mode

Used to describe data that seem to have two centers (bimodal).

Most frequently occurring value in a dataset

Range

The largest value in the data minus the smallest value, which represents the extremes of data one might encounter

Standard Deviation

• Measure of the dispersion of values from the mean

• Helps describe the normal curve

• Most frequent use measure of variation

Coefficient of Variation (CV)

Percentile expression of the mean

Sensitivity

Measure the smallest concentration of analyte of interest

Specificity

Measures only the analyte of interest

Accuracy

Nearness or closeness to the true or target value

Precision

Repeated results on the same sample that agree with one another

Diagnostic Sensitivity

Detect the proportion of individuals with the disease

More true positive results and few false negative results

Diagnostic Specificity

Detect the proportion of individuals without the disease

More true negative results and few false positive results

Confirmatory tests require high specificity

Positive Predictive Value

Chance of an individual having a given disease or condition if the test is abnormal

Negative Predictive Value

Chance an individual does not have a given disease or condition if the test is within the reference interval

Analytic Sensitivity

Ability of the test to detect small quantities of analyte

Analytic Specificity

Ability of the test to detect only the analyte of interest

Quality Control

Ensuring accuracy and precision in the laboratory

Process of ensuring that analytical results are correct

Intralab Quality Control (Internal QC)

• Analyses of control samples together with the patient specimens

• Daily monitoring of accuracy and precision

Interlab Quality Control (External QC)

• Involves proficiency testing

• Long term accuracy

• Gold standard

Goal of proficiency testing

Ensure our clinicians that patient results are accurate

Proficiency testing

Allows each laboratory to compare and evaluate test results or outcomes with those laboratories that use same methods

Control Materials

Are substances that contain an established amount of the substance being tested

Control Materials

Available in a variety of forms. They may be frozen, freeze-dried or chemically preserved.

The freeze-dried or lyophilized materials must be reconstituted

Assayed Controls

• Have predetermined target value, established by the manufacturer

• More expensive than unassayed controls

Unassayed Controls

• Also known as in-house controls

• The laboratory must establish the target value of the analyte

• Advantage is that the laboratory can produce very large volumes with exact specifications

Calibrators

• Solution with a specified defined concentration that are used to set or calibrate an instrument, kit, or system before testing is begun

• Should not be used as controls since they are used to set the instrument

• Are sometimes called standards, but the term _____ is preferred

• They usually do not have the same consistency as patient’s samples

Central Tendency

The variability of repeated measurements will be distributed around a central point or location

Normal Distribution

If many measurements are taken, and the results are plotted on a graph, the values form a bell-shaped curve as the results vary around the mean. This is called a normal distribution (also termed Gaussian distribution).