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Case 12: Public Health (Cervical Cancer and Screening), Prevalence =…
Case 12: Public Health (Cervical Cancer and Screening)
Epidemiology of Cervical Cancer
Outline the Epidemiology of Cervical Cancer
Cervical cancer is the 2nd most common cancer in women in less developed countries including SA
Cervical cancer is due to strains of the Human Papilloma Virus (HPV) known as HPV 16 and HPV 18
HPV 16 and HPV 18 cause 70% of Cervical cancers
HPV also causes Penile cancer and Genital warts
HPV is the most common Sexually Transmitted Virus
HPV-infected women have greater rates of invasive cervical cancer
HPV infection promotes the acquisition of HIV
Early treatment of HPV prevents up to 80% of cervical cancer
Determinants/Risk Factor Socio-ecological Framework
Outline the Socio-ecological Frame of Cervical Cancer caused by HPV
Biological
HPV infection (necessary for Cervical cancer)
Sex
Co-morbidities such as HIV
Behavioural
Early sexual debut
Multiple sexual partners
Smoking
Substance abuse
Long-term use of oral contraceptives
Societal
Societal norms
Gender norms
Violence such as Sexual Violence
Rape
Not using condoms
Structural
Poverty
Weak health system such:
Poor service delivery
Poor screening of Cervical cancer
Lack of education about Cervical cancer
Policies
Migrant labour systems
Levels of Prevention
List the Levels of Prevention
Primordial Prevention
Individual and Mass education
Policies aimed to address societal and structural risk factors
Primary Prevention
Health promotion and Specific protection
Reduction of risks before the occurrence of a disease or to prevent the occurrence of a disease
Secondary Prevention
Aims of early detection of asymptomatic conditions through preventative screening and tests
Tertiary Prevention
Aims to provide treatment of a symptomatic individual through medical equipment
Prevention of Cervical Cancer
Outline the Levels of Prevention used for Cervical Cancer
Primordial Prevention
Policies and system to promote:
Access to healthcare
Gender equality
Education
Reduce smoking
Primary Prevention (aimed at risk population of women aged 35 years and older)
HPV Vaccine in girls aged 9-13 years
Education on Safe sexual practices and Benefits in the delayed start of sexual activity
Male circumcision
Secondary prevention
Screening of Cervical Cancer using Cervical Cytology Smears
Liquid based Cytology (LBC)
Visual Inspection with Acetic Acid (VIA)
HPV DNA/mRNA testing
Tertiary Prevention
Prompt treatment using: Surgery, Chemotherapy and Radiotherapy
Systems Considerations
List the Considerations used for a System
Clear Strategy
Service Delivery
Age group not in regular contact with health services
Health workforce
Surveillance
Coverage
Adverse events
Monitoring cold chain
Supplies
Financing
System must be cost-effective but requires sound financing mechanism
Cost is likely to be offset by a decrease in cervical cancer int he future
Evaluation
Screening
Outline the Purpose of Screening
The Purpose of Screening is the early detection of a Disease, Precursors of the Disease, and Susceptibility to the Disease in individuals who do not show any signs of the disease
Disease that are screened for include: Breast cancer, HIV, Diabetes mellitus, and Cervical Cancer
To reduce morbidity and mortality from the disease and improve the quality of life among persons being screened
Application of a relatively simple, inexpensive test for the purpose of classifying them with respect to their likelihood of having a particular disease
Detection of people at increased risk of developing the disease, who require further evaluation
Screening
List the requirements for Screening
The requirements for screening are based on the following categories:
The Disease
The Test
The Follow-Up
The Outcome
Outline the requirements for Screening: The Disease and The Test
The Disease
The disease should be an important public health problem
The disease should be known to cause significant suffering, disability or death if detected late
An early asymptomatic stage of the disease should exist
The Test
Test should be simple and cost-effective test
Test must be Acceptable
Test must be Safe
Test must have a good Sensitivity, Specificity, PPV and NPV
It should NOT be a diagnostic test
Outline the requirements for Screening: The Follow-Up and The Outcome
The Follow-Up
The diagnosis should be confirmed
Treatment should be available
The Outcome
There is evidence that early treatment during the asymptomatic stage improves the long-term outcome
Examples of Screening
List the examples of Screening
Questions
Clinical Examination
Laboratory Tests
Genetic Tests
X-rays
Cervical Cancer
Describe why Cervical Cancer needs to be Screened for
The Disease
Important public health problem
It causes Morbidity and Mortality
It has an early asymptomatic stage
Test
Cervical Cytology Smear
Liquid based Cytology
Visual inspection with Acetic Acid
HPV DNA/mRNA test
Follow-up
Treatment
Surgery, Chemotherapy or Radiotherapy
Screening vs Surveillance
Differentiate between Screening vs Surveillance
Screening
The Goal of screening is to benefit the individual being screened, by detecting potential disease indicators
Surveillance
Surveillance is the on-going systemic collection of data
Goal of Surveillance is to provide health information about the disease or risk factors in the population
Measuring the Efficacy of Screening Tests
Efficacy of Screening Tests is measured using the contingency table
Validity of Screening Tests
List the Measurements used to Validify Screening Tests
Validity of a Screening Test measures how accurately the screening tests identifies those who have the disease and those who do not have the disease
Sensitivity
Specificity
Positive Predictive Value (PPV)
Negative Predictive Value (NPV)
Validity of the Screening Tests
Outline the Measurements used to Validify Screening Tests
Validity of a Screening Test measures how accurately the screening tests identifies those who have the disease and those who do not have the disease
Sensitivity
Sensitivity is the ability of the test to identify those who truly have the disease (True Positive)
It is the probability if the test being Positive, if the person truly has the disease
I know my patient has the disease. What is the chance the test will show my patient has it?
Specificity
Specificity is the ability of the test to exclude those who do not have the disease (True Negative)
It is the probability of the test being Negative, if the person does not have the disease
I know my patient doesn’t have the disease. What is the chance the test will show that my patient doesn’t have it?
Positive Predictive Value (PPV)
Positive Predictive Value is the probability of the person having the disease, if the test is Positive
I just got a positive test result back on my patient. What is the chance my patient actually has the disease?
Negative Predictive Value
Negative Predictive value is the probability of the person not having the disease, if the test is Negative
I just got a negative test result back on my patient. What is the chance that my patient actually doesn’t have the disease?
Measuring the Quality of a Screening Test
Outline the Measures of Screening Test Quality
Sensitivity and Specificity are the measures of Quality of a Screening Test
Sensitivity
Sensitivity = (a)/(a+c) x100%
Sensitivity = (True Positive)/ (True Positive + False Positive)
Sensitivity (True Positive Rate) is the probability of the test being Positive, if the person truly has the disease
Specificity
Specificity = (d)/(d+b) x100%
Specificity = (True Negative)/ (True Negative + False Negative)
Specificity (True Negative Rate) is the probability of the test being Negative, if the person does not have the disease
Measuring Screening Test Performance in Practice
Outline the Measurements of Screening Test Performance in Practice
PPV and NPV are the measures of Screening Test Performance in Practice
PPV and NPV are affected by Sensitivity, Specificity and Prevalence of a Pre-clinical Disease
Positive Predictive Value (PPV)
PPV = (a)/(a+b) x100%
PPV = (True Positive)/ (Tue Positive + False Positive)
(a+b) is the TOTAL number of people who tested Positive on the Screening test
PPV is the probability of the person having the disease, if their test is Positive
Negative Predictive Value (NPV)
NPV = (d)/(c+d) x100%
NPV = (True Negative)/(True Negative + False Negative)
(c+d) is the TOTAL number of people who tested Negative on the Screening Test
NPV is the probability of the person not having the disease, if their test is Negative
Screening Calculations
Calculate the Sensitivity, Specificity, PPV and NPV, and provide interpretation for each answer
Sensitivity
Sensitivity = (a)/(a+c)x100%
Sensitivity = (80)/(80+20) x100%
Sensitivity = 80%
80% of individuals with the disease will be correctly identified by the Screening Test as having the disease
Specificity
Specificity = (d)/(d+b) x100%
Specificity = (70)/(70+10) x100%
Specificity = 87.5%
88% of individuals without the disease will be correctly identified by the Screening Test as not having the disease
PPV
PPV = (a)/(a+b) x100%
PPV = (80)/(90) x100%
PPV= 89%
89% of those with a Positive Test, truly have the disease
NPV
NPV = (d)/(c+d) x100%
NPV = (70)/(70+20) x100%
NPV = 78%
78% of those with a Negative Test, truly do not have the disease
Prevalence
Prevalence = (Positive for Disease)/ (Total Number of Participants) x100%
Prevalence = (100/180) x100%
Prevalence = 55.56%
Factors which affect the Sensitivity, Specificity, PPV and NPV
Outline the factors which affect the Sensitivity, Specificity, PPV and NPV
Sensitivity and Specificity are properties of the Test
They are NOT affected by Prevalence
PPV and NPV are properties of both the Test and the Population
They are affected by the Prevalence
When the Prevalence falls:
PPV falls
NPV rises
Pap Smear
Ideally are Test would have a Sensitivity and Specificity of a 100%
For a Pap Smear:
Sensitivity = 60-94%
Specificity = 74-94%
PPV = 78%
NPV = 45%
Visual Inspection with Acetic Acid has a higher Sensitivity
Liquid Based Cytology has a lower Sensitivity
Sensitivity of a Pap Smear Screening Test will be affected by the:
Technique used to collect a Female's specimen during a Pap Smear
Specimen Transportation time
NOTE: A low PPV and Sensitivity could lead to the:
Over-treatment
Incorrectly diagnose a disease-free women as having High Grade CD
A high NPV and Specificity indicates that the:
Negative test is very reliable
And it is a good test to exclude disease
Introducing a Screening Programme
Screening Programme must reduce mortality and morbidity in practice
It must be an Acceptable Cost
Health system can cope with additional work load:
Screen
Follow-up
Treat
Train health workers
Supplies
Features of a good Screening Programme
List the features of a good Screening Programme
A good Screening Programme has Disease that are:
Important to the individual and community
Have a natural history and are Adequately understood
Have a latent or early asymptomatic stage (pre-clinical)
Has a suitable screening test
Is economically beneficial
Has an acceptable and affordable form of treatment
Successful Screening Programme
Outline the features of a Successful Screening Programme
Successful Screening Programme has:
More than 80% coverage
Appropriate Follow-up and Management of patient with positive tests
Effective links between screening, diagnosis and treatment services
High quality care
Adequate resources
Negative Consequences of Screening
Outline the Negative Consequences of Screening
Negative Consequences of Screening:
False reassurance: False Negative
Unnecessary interventions: False Positives
Complications secondary to investigations
Unnecessary treatment, adverse effects of treatment
Consequences of having a diagnosis of disease including "labelling effect"
Anxiety
Cost
Challenges in Cervical Cancer Screening
Outline challenges in cervical Cancer Screening
Challenges in Cervical Cancer Screening:
Decreased access to healthcare
Lack of equipment
Shortage of staff
Poor training of staff
Loss to follow-up
Long waiting lists for intervention
NOTE: Primordial and Primary Intervention is essential
HPV Screening
HPV screening increases the Sensitivity of the Pap Smear
Increased NPV can increase the screening interval
Cervico-vaginal self-samples specimens can increase access
HPV screening are cost-saving
Prevalence = (Positive for Disease)/ (Total Number of Participants) x100%
