Principles and Predictive Value of Screening

1
Principles and Predictive Value of Screening
2
Objectives

• Discuss principles of screening
• Describe elements of screening tests
• Calculate sensitivity, specificity and positive
  predictive value
• Discuss how a clinician can make test results
  more meaningful to the client
• Explore factors that influence clinical
  interpretation and explain how to individualize
  them for each client

3
Screening

• Objective is to reduce mortality and morbidity
  with early detection
• Screening is the application of a test to people.
  It assist with early identification of a
  disease.

4
Principles of Good Screening Programs

• Screen for health problems that
• Are important to the individual and community
• Have an acceptable form of treatment
• Has a natural history that is adequately
  understood
• Has a recognizable latent or early symptomatic
  stage
• Has a suitable screening test
• Is economically beneficial

5
The Screening Test

• Ideally should be inexpensive, easy to administer
  (low risk) and with minimal discomfort
• There should be a Gold Standard based on the
  evidence
• Results should be accurate/valid and
  reliable/reproducible/precise

6
Accurate/Valid

• Defined as
• The degree to which a variable actually
  represents what it is supposed to represent.
• Best way to assess
• Compare with a reference
• Threatened by systematic error (bias)
• Due to observer, subject and/or instrument

7
Reliable/Precise

• Defined as
• The degree to which a variable has nearly the
  same value when measured several times
• Best way to assess
• Repeated measures
• Threatened by random error
• Due to observer, subject and/or instrument

8
Predictive Value

• Is determined by Sensitivity, Specificity and the
  Prevalence of the disease
• Prevalence is defined as the number of patients
  per 100,000 population who have the disease at a
  given time

9
(No Transcript)
10
(No Transcript)
11
The sensitivity and specificity are properties
of the test. The positive and negative
predictive values are properties of both the
test and the population you test. If you use a
test in two populations with different
disease prevalence, the predictive values will
be different. A screening test is most useful if
directed to a high-risk population (high
prevalence and high predictive value).
12
How to remember

• Sensitivity "I know my patient has the disease.
  What is the chance that the test will show that
  my patient has it?
• Specificity "I know my patient doesn't have the
  disease. What is the chance that the test will
  show that my patient doesn't have it?"

13
Cont.

• PV I just got a positive test result back on
  my patient. What is the chance that my patient
  actually has the disease?
• -PV I just got a negative test result back on
  my patient. What is the chance that my patient
  actually doesn't have the disease?

14
Now to the Math
15
Using the 2X2 table you can calculate

• Sensitivity a / (ac)
• Specificity d / (bd)
• PV a/(ab)
• - PV d/(cd)
• Knowing the prevalence of the disease in the
  population is necessary for these calculations

16
Understanding Predictive Value

• Prevalence is defined as the number of patients
  per 100,000 population who have the disease at a
  given time.
• A high PV indicates a strong chance that a
  person with a positive test has the disease
  whereas a low PV is usually found in populations
  with low prevalence of the condition being
  examined. A high -PV means that a negative test
  in effect rules out the disease.

17
Effects of PrevalenceSensitivity95
Specificity95

• Populations
• Prevalence
• 0.1
• 1.0
• 2.0
• 5.0
• 50

• Predictive Value of a Positive Test
• 1.9
• 16.1
• 27.9
• 50
• 95

18
Effects of PrevalenceSensitivity99
Specificity99

• Populations
• Prevalence
• 0.1
• 1.0
• 2.0
• 5.0
• 50

• Predictive Value of a Positive Test
• 9.0
• 50
• 66.9
• 83.9
• 99

19
SpPins and SnNout

• SpPIn
• When a sign, test or symptom has an extremely
  high specificity (say, over 95), a positive
  result tends to rule in the diagnosis. For
  example, the specificity of 3 or more positive
  responses on a CAGE questionnaire in diagnosing
  alcoholism is gt99 among internal medicine
  patients. Therefore, if a person does answer
  "yes" to 3 or 4 of the CAGE questions, it rules
  in the diagnosis of alcohol dependency.
• SnNOut
• When a sign, test or symptom has a high
  sensitivity, a negative result rules out the
  diagnosis. For example, the sensitivity of the
  loss of retinal vein pulsation in diagnosing high
  intracranial pressure is 100 per cent. Therefore,
  if a person displays retinal vein pulsation, it
  rules out important increases in intracranial
  pressure.
• http//www.minervation.com/cebm2/docs/spPinsnNout.
  html

20
Website Reading

• 1. Testing a Test
• http//www.jr2.ox.ac.uk/bandolier/band3/b3-1.html
  
• By, J A Muir Gray MD FRCP (Glas) MRCGP FFCM
  Director of Health Policy and Public Health
• 2. SpPins and SnNouts
• http//www.minervation.com/cebm2/docs/spPinsnNout
  .html

21
Suggested Assignment

• Guided discussion or an individual assignment
  about a specific screening
• Exploring the natural history of the disease, the
  gold standard, different population (prevalence),
  and calculating specificity, sensitivity, PV and
  PV
• Debating the pro and cons of this screening as it
  relates to the Principles of Screening and
  Screening Test
• Interpretation of results and relaying
  information to clients

22
The End

• This presentation was created by
• Mary Beth Bigley, DrPH-c, ANP
• Assistant Professor
• Director of the Nurse Practitioner Program
• The George Washington University
• You are welcome to contact me to discuss the use
  of this material and additional learning
  activities. npamxb_at_gwumc.edu