Skip to main content
Medicine LibreTexts

2.25: Exercise 25; Quality Control

  • Page ID
    38653
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    RELATED READING: Chapter 21

    OBJECTIVE

    Upon completion of this exercise, appropriate discussion, and related readings, the student will be able to:

    1. Gain an understanding and working knowledge of the principles of quality control (QC) in clinical chemistry.
    2. Gain experience in plotting quality control data.
    3. Perform statistical calculations and interpret QC data.

    PRINCIPLE

    In general, quality control of laboratory analyses is performed by assaying quality control pool samples on a daily basis and maintaining a record of these values. After a sufficient number of measurements (i.e., >20 obtained on consecutive days with the same control), an expected target range of acceptable results can be established. A common way of doing this is to determine the mean and standard deviation (S.D.) of the data, then set the acceptable range as the mean ±2 S.D. This will include 95% of the data, and we would expect a value outside of this range to occur by chance in only 1 of 20 subsequent analyses. When a result for a quality control pool falls outside the target range, the medical technologist must question the validity of the run and respond in some appropriate fashion, either by repeating the control analysis or by repeating the entire run after trouble-shooting the analyzer.

    GLOSSARY

    ±2 S.D. limits - also called the 4 S.D. target interval, defines the range that includes all data between + 2 S.D. to - 2 S.D. about a mean value.

    Out of control - a term indicating that the results from one or more quality control analyses falls outside of the “accepted” target range (± 2 S.D., for example).

    MATERIALS

    • Calculator

    PROCEDURE

    1. Calculate the mean, standard deviation, and coefficient of variation for the following results (mg/L) obtained on a quality control pool for calcium analysis. Use the table provided on the data sheet for calculation.

    101, 102, 100, 98, 99, 101, 103, 100, 95, 98, 98, 100, 101, 99, 102, 103, 102, 104, 102, 104, 105, 105, 99, 95, 97, 99, 104, 95, 103, 96.

    1. Consider the following data obtained from the analysis of a quality control pool on 20 consecutive days for five common laboratory analytes. Plot the graph on the Levy-Jennings plotsprovided on the data sheet. On each graph fill in the calculated mean (target value) and the + and - 2 S.D. range values.

    glucose (mg/L): 940, 990, 860, 910, 900, 840, 980, 980, 1180, 780, 1020, 930, 910, 780, 950, 1050, 800, 850, 950, 820; 4 S.D. target interval 900-1100 mg/L.

    chloride (mmol/L): 104, 101, 100, 104, 103, 102, 105, 104, 102, 105, 103, 102, 105, 104, 105, 106, 106, 107, 109, 108; 4 S.D. target interval 98-108 mmol/L.

    potassium (mmol/L): 4.2, 4.4, 4.3, 4.9, 4.4, 4.5, 4.2, 4.4, 4.9, 4.5, 4.5, 5.1, 4.9, 4.7, 5.2, 4.9, 4.5, 4.7, 4.9, 4.7; 4 S.D. target interval 3.5-5.3 mmol/L.

    calcium (mg/L): 86, 98, 99, 91, 109, 100, 93, 97, 95, 95, 110, 100, 90, 101, 89, 93, 103, 106, 91, 87; 4 S.D. target interval 85-105 mg/L.

    sodium (mmol/L): 144, 139, 146, 142, 155, 139, 151, 143, 146, 132, 142, 147, 143, 144, 134, 139, 145, 148, 136, 142; 4 S.D. target interval 136-148 mmol/L.

    1. Identify trends, shifts, and out of control data. Circle all out of range data, that is, those results >±2 S.D. from the mean or target value.

    Discussion Questions

    Examine each of the Levy-Jennings plots, and answer the following questions.

    1. Which of the methods appear to be in overall good control?
    2. Which of the methods may have accuracy and/or precision problems?
    3. What decisions do you think might be made on day 20 for each of the analytes?
    4. These Levy-Jennings plots review only one quality control pool, although 2 pools are usually analyzed each day. What would you do if the low (“normal”) control pool was within the target range but the abnormal control pool was outside the established limits?
    DATA SHEET, EXERCISE #25

    NAME: ___________

    DATE: ___________

    RESULTS

    Calcium value (Xi), mg/L Xi – X (Xi – X)2
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         

    CALCULATIONS

    N = 31

    \(\Sigma\; X_{i}\) = __________

    \(\Sigma (X_{I} – X)^{2}\) = __________

    Mean = \(\frac{X + \Sigma X_{i}}{N}\) = __________mg/L

    \(S^{2} = \frac{\Sigma(X_{i} – X)^{2}}{N-1}\) = _____ S.D. = standard deviation = _____mg/L

    %coefficient of variation = % C.V. = S.D. \(\frac{(100 \%)}{mean}\) = _________%

    95% interval = __________mg/L


    2.25: Exercise 25; Quality Control is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

    • Was this article helpful?