This document defines key terms related to reference values and establishes guidelines for determining reference values. It states that reference values are needed to interpret clinical laboratory results by comparing a patient's measurements to values from healthy or relevant disease populations. It outlines conditions for establishing reference values, such as clearly defining reference groups, using standardized methods, and accounting for factors like age and sex. The document recommends excluding individuals from reference values who have diseases, risk factors, medical treatments, or physiological states that could impact measurements. It emphasizes reference values should be established from random samples and may require separate values for different groups.

1. ESTABLISHING AND USE OF
REFERENCE VALUES

2. Definition of terms
• Reference value – A value obtained by observation or
measurement of a particular type of quantity on a
reference individual
• Reference individual – An individual selected as basis for
comparison with individuals under clinical investigation,
through the use of defined criteria
• Sensitivity (Clinical) – The proportion of subjects with
disease who have positive results
• Specificity (Clinical) – The proportion of subjects without
disease who have negative test results

3. ESTABLISHMENT OF REFERENCE VALUES
• Data collected during medical investigations must be
interpreted by comparison with reference data
• The interpretation of clinical laboratory data confirm
conditions of the patient’s observed symptoms or rule out
signs that do not fit the patterns characterizing a set of
alternative diseases
• Reference values are therefore needed for all tests
performed in clinical laboratory from healthy individuals
and from patients with relevant diseases

4. ESTABLISHMENT OF REFERENCE VALUES
Conditions necessary to make the comparison of patient’s
laboratory results with reference values :
– All groups of reference individuals should be clearly defined
– The patients examined should resemble sufficiently the reference
individuals in all aspects other than those under investigation
– The condition under which the samples were obtained and
processed for analysis should be known
– All quantities compared should be of the same type
– All laboratory results should be produced with the use of adequately
standardized methods under sufficient analytical quality control
– The stages in the pathogenesis of the diseases that are the
objectives for diagnosis should be stated
– The diagnostic sensitivity and specificity, prevalence and clinical
costs of misclassification should be known for all laboratory tests
used

5. ESTABLISHMENT OF REFERENCE VALUES
• Reference values are results of a certain type of quantity
obtained from a single individual or group of individuals
corresponding to a stated description which must be
spelled out and made available for use by others
• Reference values was associated with health-associated
reference values and termed as normal values
• Subject based reference values are previous values from
the same individual obtained when the individual was in a
state of health
• Population-based reference values are those obtained from
a group of systematically defined reference individuals and
are the values termed as reference values commonly used

6. ESTABLISHMENT OF REFERENCE VALUES
• Reference individuals should be a random
sample of all the individuals in the parent
population who fulfill the selection criteria
• Often separate reference values for sex and
age groups are necessary because analytes
vary significantly among different age and
gender groups
•

7. SELECTION OF INDIVIDUALS FOR HEALTH
ASSOCIATED REFERENCE VALUES
• Examples of Exclusion Criteria for Health-
Associated Reference Values:
• Diseases
– Risk factors: Obesity, Hypertension, Risk from
occupation or environment, Genetically determined
risks
– Intake of pharmacological active agents: Drug
treatment for disease, Oral contraceptive, Drug abuse,
Alcohol, Tobacco
– Specific physiological states: Pregnancy, Stress,
Excessive exercise

9. Diagnostic Value
• A test used to diagnose disorders has four possible
outcomes:
– True positive (TP): a positive result for a patient who has
the condition (correct result)
– False positive (FP): a positive result for a patient who
does not have the condition (wrong result)
– True negative (TN): a negative result for a patient who
does not have the condition (correct result)
– False negative (FN): a negative result for a patient who
has the condition (wrong result)

10. The Four Possible Outcomes of a Diagnostic Test
Positive Negative TOTAL
Diseased TP FN TP + FN
Healthy FP TN FP + TN
TOTAL TP + FP FN + TN TP + FP + FN + TN
TP - true positive
FP - false positive
TN - true negative
FN - false negative

12. Sensitivity and Specificity
Exercise:
You are provided with 300 serum samples
from a population with 120 HIV infected
individuals and a new screening test to be
validated using the provided serum samples.
If the kit tested 4 false positive and 15 false
negative serum samples, calculate the Sensitivity,
Specificity, Positive predictive vale and Negative
predictive value? What is the efficiency of the
new test kit.

13. Sensitivity and Specificity

14. Efficiency - Accuracy

15. Prevalence

16. Predictive values

17. Assignment