Consider an example of estimating the measurement uncertainty for a blood glucose test performed in a medical laboratory. We will use the following uncertainty components:
- Repeatability of the Measurement (within-laboratory variation): 2.0 mg/dL
- Reproducibility of the Measurement (between-laboratory variation): 1.5 mg/dL
- Uncertainty in Calibration: 0.8 mg/dL
Step 1: Identify the Sources of Uncertainty: We have identified three sources of uncertainty: repeatability, reproducibility, and calibration uncertainty.
Step 2: Quantify the Uncertainty Components: a) Repeatability (standard deviation of repeated measurements within the laboratory): 2.0 mg/dL b) Reproducibility (standard deviation of measurements between different laboratories): 1.5 mg/dL c) Calibration Uncertainty: 0.8 mg/dL
Step 3: Combine Uncertainty Components: To calculate the overall measurement uncertainty, we use the root-sum-square (RSS) method to combine the uncertainties:
Combined Standard Uncertainty (uc) = √(Repeatability^2 + Reproducibility^2 + Calibration Uncertainty^2)
uc = √(2.0^2 + 1.5^2 + 0.8^2) ≈ √(4.0 + 2.25 + 0.64) ≈ √7.89 ≈ 2.81 mg/dL
Step 4: Express Uncertainty: As we want to report the uncertainty with 95% confidence, we use a coverage factor (k) of approximately 2 for a coverage probability of 95%.
Expanded Uncertainty (U) = uc * k = 2.81 * 2 ≈ 5.62 mg/dL
Step 5: Document the Uncertainty Estimate: The measurement uncertainty for the blood glucose test is approximately ±5.62 mg/dL at a 95% confidence level.
Step 6: Monitor and Update: The laboratory should regularly monitor and reassess the uncertainty estimation to account for any changes in the test method, equipment, or environmental conditions.
Please note that the uncertainty estimation process may vary based on the specific laboratory setup and the measurement method used. It’s essential to follow the relevant standards and guidelines, such as ISO 15189, to ensure accurate and reliable uncertainty estimation.
About the author
Dr. Sambhu Chakraborty is a distinguished consultant in quality accreditation for laboratories and hospitals. With a leadership portfolio that includes directorial roles in two laboratory organizations and a consulting firm, as well as chairmanship in a prominent laboratory organization, Dr. Chakraborty is a respected voice in the field. For further engagement or inquiries, Dr. Chakraborty can be contacted through email at director@iaqmconsultants.com and info@sambhuchakraborty.com. Additional resourcesand contact information are available on his websites, https://www.quality-pathshala.com and https://www.sambhuchakraborty.com, or via WhatsApp at +919830051583