rev.ANCH121_013 - Quality Control Chart

• Quality Control Chart


  • Used to observe values of control materials over time to determine reliability of the analytical method
  • Utilized to observe and detect analytical errors such as inaccuracy and imprecision
• Gaussian Curve (Bell-shaped Curve)


  • Obtained by plotting the values from multiple analyses of a sample
  • When the data set can be accurately described by the standard deviation and mean
  • A population probability distribution that is symmetric about the mean
  • Focuses on the distribution of errors from the analytical method rather than the values from a healthy or patient population
  • The total are under the curve is 1.0 or 100%
  • Acceptable for MedTechs (positive or negative 2SD)
• Cumulative Sum Graph / Q-Sum


  • Calculates the difference between QC results and the target means
  • Common method: V-Mask
  • Gives the earliest indication of systematic assays (trend) and can be used with the I3s rules
  • Very sensitive to small, persistent errors that commonly occur I the modern, low calibration – frequency analyzer
  • Results are out of control when the slope exceeds 45º or a decision pN 2.7SD is exceeded
• Youden or Twin Plot


  • Used to compare results obtained on a high and low control serum from different laboratories
  • Displays the results of the analyses by plotting the mean values for one specimen on the y-axis (ordinates) and the other specimen on the x-axis (abscissa)
  • The points falling from a center but on the 45º line suggest a proportional error, and points falling from the center but not on the 45º line suggest a constant error
• Shewhart Levey-Jennings Chart


  • Most widely used QC chart in the clinical laboratory
  • Allows the laboratories to apply multiple rules without the aid of a computer
  • A graphic representation of the acceptable of variation in the results
  • Easily identifies random and systematic method
• Errors observed on LJ Chart


  • Trend – formed by control values that either increase or decrease for 6 consecutive days (Main Cause: Deterioration of Reagent)
  • Shift – formed by control values that distribute themselves on one side or either side of the mean for 6 consecutive days (Main cause: improper calibration of the instrument)
  • Outliers - control values that are far from the main set of values
• Westgard Control Chart


  • 12s Used a rejection or warning rule when one control results exceeds the mean of +/- 2SD it is for screening purposes
  • 13s One control results exceeds the means +/- 3SD; it is effective in determining random error
  • 22s the last 2 control results exceed either the mean +/- 2SD: respond most often to systematic error
  • 41s the four consecutive control results exceed either mean: +/- 4SD respond to systematic error
  • R4s the range or difference between the highest and lowest control results within an analytical run exceed: +/- 4SD responds to both random errors or increased imprecision
  • 10x. Ten consecutive results are on the same side of the target mean: systematic errors