Medical Test Sensitivity and Specificity

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Questions and Answers

What is the primary focus of Quality Control activities?

To establish standard procedures for sample collection

To evaluate a product or work result (correct)

To define criteria for acceptable samples

To ensure processes meet system objectives

Which of the following is a method for preventing errors during testing?

Conduct tests based on verbal instructions

Record results in an electronic format

Randomly select samples for testing

Follow safety precautions (correct)

What do Westgard multi-rules help to achieve in Quality Control?

Determine the cost-effectiveness of testing

Guide acceptance or rejection of patient results (correct)

Define acceptable sample sizes

Set standard operating procedures for laboratories

Which example represents a post-analytic error?

Transcribing results inaccurately Signup and view all the answers

What is a potential issue when a confirmed HIV-positive patient is re-tested and receives a negative result?

Errors in patient/client identification Signup and view all the answers

What is the importance of understanding the characteristics of non-affected persons in relation to testing?

It may affect cross reactivity due to diverse antibodies. Signup and view all the answers

Which statement is true about the specificity of a test?

Specificity is an intrinsic characteristic of the test. Signup and view all the answers

In the context of test cut-offs, what does a real case analysis reveal?

False positives and false negatives can occur with real cases. Signup and view all the answers

Which groups are most affected by sensitivity and specificity in testing?

Panels of affected and negative individuals. Signup and view all the answers

What can be inferred about a population with endemic malaria in relation to testing?

They may experience increased false positives due to polyclonal hypergammaglobulinemia. Signup and view all the answers

What does the cut-off in a test determine?

It establishes the boundary between sick and well individuals. Signup and view all the answers

Why is laboratory training important for field epidemiologists?

It helps them accurately interpret sensitivity and specificity. Signup and view all the answers

What does the potential for false positives imply for the specificity of a test?

It indicates that specificity may be compromised. Signup and view all the answers

What is the primary goal when testing non-affected individuals?

To estimate the proportion of non-affected individuals that are negative Signup and view all the answers

Which individuals should ideally be tested for assessing the specificity of a leishmaniasis test?

Individuals without signs of infection and at low risk Signup and view all the answers

How is the specificity of a rapid test for leishmaniasis calculated?

TN/(TN + FP) Signup and view all the answers

What was the specificity percentage of the rapid test for leishmaniasis mentioned?

94% Signup and view all the answers

Which of the following groups should be excluded from testing when estimating specificity?

Individuals already diagnosed with leishmaniasis Signup and view all the answers

What does a high specificity indicate about a rapid test?

It accurately identifies individuals without the disease Signup and view all the answers

What is meant by a 'true negative' in the context of specificity?

A person tested negative and does not have the disease Signup and view all the answers

Which of the following factors might influence the specificity of a test?

The prevalence of disease in the population Signup and view all the answers

What is the definition of analytical specificity?

The ability to distinguish one analyte apart from similar substances Signup and view all the answers

What does specificity indicate in terms of a diagnostic test?

The ability to correctly identify non-affected individuals Signup and view all the answers

Which of the following factors is considered when estimating the specificity of a test?

The identification of non-affected individuals Signup and view all the answers

What must be obtained to ensure the specificity of a test is estimated accurately?

A wide panel of samples representing non-affected individuals Signup and view all the answers

What does a high specificity in a test imply?

Few false positives in identifying healthy individuals Signup and view all the answers

What is a common misconception about the sensitivity of a test?

It measures how well a test identifies true negatives Signup and view all the answers

Why is it difficult to estimate the specificity in diverse populations?

Finding a representative sample of unaffected individuals is challenging Signup and view all the answers

What is usually underestimated when evaluating test sensitivity?

The varied antigenic characteristics in the local epidemiology Signup and view all the answers

What is the primary aim of a formal quality system in a laboratory?

To prevent mistakes and ensure accuracy of results Signup and view all the answers

Which of the following is NOT a component of quality assurance measures?

Replicate analyses of test results Signup and view all the answers

What are quality control measures primarily concerned with?

Specific analytical tests Signup and view all the answers

What is the purpose of joining proficiency testing schemes?

To assess and improve testing accuracy Signup and view all the answers

Which of the following is an essential part of quality system documentation?

Document approval processes Signup and view all the answers

How does occurrence management contribute to quality control?

Through corrective actions for addressing errors Signup and view all the answers

What role does customer satisfaction monitoring play in quality systems?

It ensures continuous improvement of services Signup and view all the answers

Which component is part of the necessary infrastructure for a quality system?

Regular audits of all processes Signup and view all the answers

Which of the following best describes total quality management (TQM)?

A methodology that addresses all aspects of laboratory practice Signup and view all the answers

What is the relevance of external quality assessments?

They provide insights into internal improvement areas Signup and view all the answers

Why is equipment maintenance important in a quality system?

It ensures accuracy and reliability of results Signup and view all the answers

What is the impact of proper hiring and training in a quality system?

It contributes to maintaining high quality standards Signup and view all the answers

Which component is primarily focused on managing documentation and records?

Quality system documents Signup and view all the answers

What is a fundamental goal of a health laboratory?

To facilitate accurate patient laboratory results Signup and view all the answers

Study Notes

### Specificity

Specificity is an intrinsic characteristic of the test and is estimated on a population of non-affected individuals.
The diversity of antibodies to various other antigens in the population may affect cross-reactivity (e.g., If malaria is endemic, polyclonal hyper gammaglobulinemia may increase the proportion of false positives).

Identifying the Cut-off of a Test

In an ideal case, there is a clear distinction between sick and well individuals based on the test results.
In a real case, there may be false positives and false negatives, making it challenging to determine a definitive cut-off.

Sensitivity and Specificity

Sensitivity refers to the ability of a test to correctly identify individuals who are affected by the disease.
Specificity refers to the ability of a test to correctly identify individuals who are not affected by the disease.
To laboratory specialists, individuals that would need to be tested but are not infected are the most important.
It is crucial to estimate the sensitivity and specificity of a test by testing both affected and non-affected individuals.

Estimating the Specificity of a Rapid Test for Leishmaniasis

Identify persons without leishmaniasis using a gold standard test and by targeting persons with low risk of infection.
Obtain a wide panel of samples that are representative of the population of individuals without leishmaniasis.
Estimate the proportion of persons without leishmaniasis that are negative with the rapid test.

Calculating Specificity

Specificity is calculated as the number of true negatives divided by the total number of non-affected individuals.
A 95% confidence interval can be estimated to assess the reliability of the specificity calculation.

Factors Influencing Test Specificity

Quality Assurance (QA) system: A formal QA system in the laboratory can prevent mistakes by implementing quality assurance measures, quality control checks, thorough documentation, efficient record keeping, and regular audits.
Quality Control (QC) system: QC measures apply to each analytical test in the laboratory by using reagent blanks, verified standard solutions, check samples, blind samples, replicate analyses, and control charts.

Essential Components of a Quality System

Organization: A quality policy and standards should be established, ensuring sufficient resources and a culture committed to quality.
Personnel: Hiring, training, supervision, and performance management are critical aspects of personnel quality.
Equipment: Selection, acquisition, installation, initial calibration, maintenance, troubleshooting, service and repair, and disposition should be carefully managed.
Purchasing and Inventory: Procurement, receiving, storage, inventory management, and record-keeping are essential for maintaining a reliable supply of materials.
Process Control: Standardized operating procedures should be followed for sample management, quality control, and specimen management.
Documents and Records: Standardized forms, document approval and distribution, storage and retrieval, and destruction of documents all contribute to a robust system.
Information Management: The flow of information, data collection and management, patient privacy and confidentiality, and computer skills are all crucial aspects of information management.
Occurrence Management: Written procedures for addressing errors, occurrence records, and occurrence reporting are important for identifying and mitigating errors.
Assessment: Internal audits, self-evaluation, and external quality assessment are important for identifying and addressing areas for improvement.
Process Improvement: Ongoing data collection, improvement measures, and monitoring are essential for continuous improvement.
Service and Satisfaction: Monitoring customer satisfaction, process improvement, and rewards contribute to a successful system.
Facilities and Safety: Testing and storage areas should be appropriately equipped, and safety practices, procedures, and records should be meticulously maintained.

Total Quality Management (TQM)

TQM addresses all areas of laboratory practice, encompassing lab service and resources.

Purpose of Health Laboratory

Provide patient laboratory results for:
- Diagnostic purposes
- Prognostic purposes
- Monitoring treatment
- Monitoring disease outbreaks

Characteristics and Aspects of Quality Assurance

Quality assurance (QA) encompasses activities to ensure processes are adequate for a system to achieve its objectives.
Quality control (QC) encompasses activities to evaluate a product or work result.
Three phases affect useful patient results: pre-analytical, analytical, and post-analytical.

Quality Control Material

QC material is used to assess the analytical phase, determining the acceptance or rejection of patient results.
Two or three control materials should be used to ensure accuracy and precision.

QC Rules

QC rules guide the acceptance or rejection of patient results based on QC results, using 95% confidence limits or Westgard multi-rules.

Errors During Testing

Perform and review QC regularly.
Follow safety precautions.
Conduct tests according to written procedures.
Interpret test results correctly.

Post-Analytic Errors

Examples include:
- Clerical errors (transcription errors, illegible reports, reports sent to the wrong location, information system not maintained, wrong reference ranges, timely reporting issues, confidentiality breaches)
- Analytical errors (wrong specimen, incorrect identification, improper test method, contaminated reagent, outdated reagent, incorrect specimen handling, incorrect temperature, incorrect dilution, incorrect machine maintenance, incorrect calculations, software failure, misinterpretation of results)
- Specimen errors (incorrect specimen, incorrect collection procedure, specimen not collected properly, insufficient quantity, insufficient volume, delay in processing, contamination, improper storage of specimen, specimen not labelled properly, incorrect transport of specimen)

Preventing and Detecting Errors After Testing

Re-check patient/client identifier.
Write legibly.
Clean up and dispose of contaminated waste.
Package EQA specimens for re-testing, if needed.

Example Exercise: Patient Result Outside Expected Range

A previously confirmed HIV-positive patient is being re-tested by HIV Rapid Test and the test results are negative. This could be due to antigenic characteristics of the pathogen in the area, as the test may not be prepared with antigens specific to the population of pathogens in the area.

Definition of Analytical Specificity

Analytical specificity refers to the ability of a test to distinguish one analyte from similar substances.
A test must have analytical specificity before it can be considered specific for a disease.
Specificity is the probability of a negative test result given the absence of disease. It measures how well a test correctly identifies uninfected individuals. A specific test will rarely misclassify people without the disease as infected (few false positives).

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Description

This quiz explores the concepts of sensitivity and specificity in medical testing. It examines how these characteristics impact the identification of affected and non-affected individuals and the challenges presented by false positives and negatives. Test your knowledge on determining cut-off points and the factors influencing test accuracy.