Clinical Chemistry I Week 07

Clinical Chemistry I - 0202304

• Course instructor is Mohammad QABAJAH
• Email address is [email protected]
• Course code is 0202304

Week 07 - Reference Values

• Objectives:
  • Explain the need for establishing reference intervals for clinical tests.
  • Detail conditions for valid comparison of lab results with reference values.
  • Define exclusion criteria for creating health-related reference values.
  • Detail partitioning criteria for subgrouping reference groups.
  • Explain parametric and non-parametric methods for determining interpercentile intervals.
  • Discuss the transferability of reference values, addressing prerequisites and solutions.
  • Differentiate between reference values and reference intervals.

Decision Making: Comparison

• Medical history, physical exams, and supplemental tests lead to a diagnosis.
• If patient's attributes resemble a known disease, that disease is a potential diagnosis.
• If not, exclude that diagnosis.

In Laboratory: Decision Making by Comparison

• Reference values:
  • Obtained from healthy individuals.
  • Obtained from patients with diseases.
• Reference values in laboratory:
  • Subject-based: from single individual
  • Population-based: from a group of individuals
• Reference values must have a clearly defined description.

Comparing Observed Data

• Reference values are collected from various groups.
  • Healthy people
  • Undifferentiated hospital population
  • People with typical diseases

Conditions to Ensure Proper Comparison

• Reference individuals' characteristics must be clearly outlined.
• The patient characteristics should be similar to the reference individuals.
• The conditions associated with the patient's lab tests must be known
• All data measurements should be the same type
• Standardized methods should be used across all results
• Clinical sensitivity, clinical specificity, and prevalence should be known

Selection of Reference Individuals

• Ideally, the entire population
• Random sampling, as large as possible (at least 120)

Exclusion Criteria

• Diseases
• Risk Factors (Obesity, Hypertension, occupation/environmental risks, genetically determined risk factors)
• Intake of pharmacologically active agents
• Specific Physiological States (Pregnancy, Stress, Excessive exercise)

Subgrouping of the Reference Group

• Factors to consider for subgrouping:
  • Age
  • Gender
  • Physiological factors (Stage in menstrual cycle, stage in pregnancy, physical condition)

Specimen Collection

• Preanalytical standardization protocols are essential.
  • Preparation of individuals before collection.
  • Correct specimen collection techniques.
  • Handling of the sample before analysis.

Analytical Procedures and Quality Control

• Defining components of the analytical process is crucial.
  • Analysis method, including details on equipment.
  • Reagents
  • Calibrators
  • Types of raw data
  • Calculation methods
  • Quality control
  • Reliability criteria

Statistical Treatment of Reference Values

• Statistical Treatment of Reference Values:
  • Dividing reference values into appropriate groups
  • Evaluating the distribution of each group
  • Determining reference limits
  • Identifying outliers

Inspection of Distribution

• Display frequency distribution graphically (e.g., histogram)
• Identify patterns like unimodal, bimodal, or polymodal distributions
• Analyze the shape of the distribution
• Identify outliers (highly deviating values)

Determination of Reference Limits

• Reference interval: Upper and lower reference limits representing a specific proportion (e.g., 95%) of the population's values.
• Range: calculated as higher value minus lower value.
• Clinical decision limits help distinguish between groups (e.g. healthy, disease-specific individuals).

Identification of Outliers

• Assume a Gaussian distribution.
• Using Interquartile Range (IQR): Outliers are values that are significantly lower or higher than the expected range based on the IQR.

Quartile Formula

• Formulae for calculating the lower quartile (Q1), middle quartile (Q2) and the upper quartile (Q3).

Interquartile Range Formula

• Calculate the Interquartile range (IQR) by subtracting the lower quartile (Q1) from the upper quartile (Q3).

Outliers Formula

• Calculates limits for outliers based on values calculated using the quartile formula and the interquartile range.

Determination of Reference Limits

• Reference limit: Values that separate populations
• Clinical decision limit: Values defining critical levels for interventions, often based on statistically significant differences.
• Reference range vs. reference interval: Clear differentiation is crucial in clinical decision-making.
• Reference interval: a broader range of values encompassing a large percentage (e.g., 95%) of the reference population

Interpercentile Interval

• Simple, widely used method for estimating reference ranges, following recommendations of IFCC.
• Parametric techniques use mean and standard deviation (SD) to calculate 2.5th and 97.5th percentiles
  • Formula for lower limit and upper limit of interval
• Non-parametric methods don't assume a specific distribution shape but use rank, percentiles, and confidence intervals.

Transferability/Transference of Reference Values

• Establishing comparability is required for using reference data from one lab in another
• The parameters considered are:
  • Population comparability (no significant ethnic, social or environmental differences).
• The factors considered for analytical performance are:
  • Proper selection and preparation
  • Identical and valid analytical methods
  • Valid external quality controls (EQCs)
  • Necessary mathematical methods when results cannot be directly compared