Screening is a tool used to identify undiagnosed cases of disease in a population using rapid tests. The objectives of screening are to reduce mortality and morbidity through early detection and treatment, and to maintain or enhance production through better disease management. Screening aims to separate healthy individuals who likely have a disease from those who do not, though a positive screening test requires more diagnostic testing. Effective screening involves both diagnostic testing and treatment components, and aims to identify existing disease in asymptomatic individuals. The only valid measure of a screening program's effectiveness is its impact on disease-specific mortality rates.

1. Screening
A Tool of Surveillance
To identify undiagnosed cases of
disease using rapid test
Bhoj R Singh
Section of Epidemiology, CADRAD, IVRI, Izatnagar-243122, India

2. Objectives/ Purpose
• Objective:
• To reduce mortality and/or morbidity by early detection and treatment.
• To Maintain or enhance production through better management.
• Purpose
– To identify undiagnosed cases of disease
– Detect outbreaks/ disease/ production losses and causes
– Detect animal health threats
– Detect infectious cases (case finding)
– Monitor trends in a target population
– Monitor exposed individuals for symptoms
– Monitor treated individuals for complications
– Direct animal health interventions
– Evaluate animal health interventions
– Generate hypotheses for further evaluation
• Aim
– To separate apparently healthy individuals that probably have a disease from those that probably
do not.
• Does not Intend for
– Diagnosis of disease at individual level, testing positive in screening test needs more
confirmatory tests/ diagnostic tests.

3. Approaches
• Population-level screening/ Mass screening
– National level policy decision to offer mass screening to a whole sub-group
of a population
• Measurement of pesticide levels / antibiotic levels in animals/ their products/
feeds/ fodder
• Tuberculin testing
• PPR antibodies in Goats
• Strategic screening
– Targeted in specific areas where disease cases may be present eg. For FMD
in a territory of 5 Km area; Bird flue in a district, FMD in vaccinated
animals etc.
• Prescriptive screening/ Individual-level screening/ Case finding
– Occurs at the individual patient-veterinarian level
– Also a component of the disease control.
– Focus is on identifying existing disease in patients who don’t know they
have it, e.g., Brucella testing in a herd or an individual animal before
purchasing

4. Components
• Effective screening involves both
diagnostic and treatment components
• Screening differs from diagnostic testing:
Screening Diagnosis
In Healthy/ non-patients In Patients
Non-diagnostic intent Diagnostic intent
Used when diseases prevalence in very low or low When disease prevalence
is low or high

5. Phases in screening
• Preclinical phase (PCP): the period between early
detection by screening and the clinical diagnosis.
Pathology begins/ infection establishes
Diseases detectible
PCP LT
Disease diagnosed and is in typical clinical form
Lead Time (LT):
• Equals the amount of time by which treatment is advanced or made
“early”.
• Not a theory or statistical artifact but what is expected and must
occur with early detection.
• Does not imply improved outcome!!
• Necessary but not sufficient condition for effective screening.

6. Characteristics of Screening tests
a) Sensitivity (Se) (Prob T+|D+)
a) the proportion of cases with a positive screening test among all
individuals with pre-clinical disease.
b) Want a highly Se test in order to identify as many cases as
possible…… but there’s a trade off with……the specificity should not
be too low
• b) Specificity (Sp) (Prob T-|D-)
the proportion of individuals with a negative screening test result
among all individuals with no pre-clinical disease.
The feasibility and efficiency of screening programs is acutely sensitive to
the positive predictive value which is often very low due to the very
low disease prevalence.
*Imperfect Sp affects many (the healthy), whereas an imperfect Se
affects only a few (the sick).

7. Evaluation of Screening results
– Compare disease-specific mortality rate (DSMR)
between those randomized to screening and
those not
– Eliminates all forms of bias (theoretically)
– But, problems of:
• Expense, time consuming, logistically difficult,
contamination, non-compliance, ethical concerns,
changing technology.
– Can also evaluate screening programmes using
Cohort and Case-control studies, but they are
difficult to do and very susceptible to bias.

8. The only valid measure of Screening
Disease-specific Mortality Rate (DSMR)
the number of deaths due to disease
Total person-years experience
– The only gold-standard outcome measure for screening
– Not affected by lead time
– when calculated from a Randomized control trials (RCT) - not
affected by compliance bias or length-time bias.
– However, there can be problems with the correct assignment of
cause of death (hence some researchers advocate using only all-
cause mortality as the outcome).

9. Biases in Screening
• Observational studies and especially survival data are acutely
sensitive to:
• 1. Compliance bias (Selection bias):
• Volunteers or compliers are better educated and more health conscious – thus
they have inherently better prognosis
• 2. Lead-time bias
• Apparent increased survival duration introduced by the lead time that results
from screening.
• Screen-detected cases survive longer event without benefit of early
treatment.
• 3. Length-time bias
• Screening preferentially identifies slower growing or less progressive cases
that have a better prognosis.
• Length-time bias – cases with better prognosis detected by screening

10. Pseudo-diseases and over diagnosis
• Over-diagnosis
– Limited malignant potential
– Extreme form of length-biased sampling
– e.g., Pap screening and cervical carcinoma
• Competing risks
– Cases detected that would have been interrupted by an unrelated
death
– e.g. Prostate Cancer and cardiovascular disease (CVD) death
• Serendipity
– Chance detection due to diagnostic testing for another reason
– e.g. PSA (Prostate specific antigen) and prostate CA (Cancer), FOBT
(Faecal occult blood test) and Colo-rectal cancer (CR CA)

11. Assessing the feasibility of screening
• Burden of disease
– Effectiveness of treatment without screening
• Acceptability
– Convenience, comfort, safety, costs (= compliance)
• Efficacy of screening
– Test characteristics (Se, Sp)
– Potential to reduce mortality
• Efficiency
– Low PVP (Predictive value of Positive) (PVN, predictive value of
negative))
– Risks and costs of follow-up of test positives
– Cost-effectiveness
• Balance of risks (harms) vs. benefits

12. Three questions before screening
• Should we screen? (scientific): Efficacy
• Can we screen? (practical): Effectiveness
• Is it worth to do it? (scientific, practical, policy,
political): Cost effectiveness