9.3 Using data to make a diagnosis

Blood tests and other diagnostics are important for health practitioners to be able to make informed evaluations of patients. Collection of this data is central to evidence-based medicine.

So, imagine you have a blood test.

A typical readout from a whole blood exam. Analytes are displayed with their measured value with an indication whether it is high or low compared to a reference value.
Figure 9.2: An example of the typical data output from a whole blood exam. The values for the different analytes are provided and are also annotated either “L” for being lower than the corresponding reference range or “H” for being higher than the corresponding reference range. Note the units of concentration are also provided given. IU/L indicates international units per litre. An international unit is an arbitrary amount of a substance agreed upon by scientists and doctors

How does your clinician interpret this data? They will look at the concentration of the analytes on the left and compare them to the range of values or threshold on the right, which represent the ‘normal’ values. Any analyte outside the range is potentially indicative of a problem or abnormality and will help form a diagnosis. There are two important questions to consider here:

  • How do we know if a measurement is normal? How are normal ranges of data and threshold determined? Data collected from patients will exhibit variability from patient to patient, so it is important to understand the amount of variability there is in this data across the population to know if a measure is likely to be normal or abnormal.
  • How reliable are the measurements? How do we know the data collected for an individual patient is reliable?

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