Screening of Disease - NEET PG PSM

Screening of disease is an important topic for the NEET PG exam because it is a crucial aspect of preventive medicine. Screening tests are used to identify individuals who are at an increased risk of developing a disease, even if they don't have any symptoms. 

By identifying these individuals early, appropriate interventions can be initiated to prevent or delay the onset of the disease, or to manage it more effectively.

This is essential for ensuring public health and improving patient outcomes, and therefore is a key area of focus for medical professionals, including those who are preparing for the NEET PG exam.

In this blog, we’ll cover all the important aspects of screening of disease. Read on.

Screening Vs Diagnosis

Screening	Diagnosis
Applied on apparently healthy people	Applied on cases
Cheap & rapidly applied tests	Time-consuming & expensive
Results are arbitrary & final	Results are not final
Inaccurate	Accurate
Never a basis of treatment	Form a basis of treatment

Important Information

• Why to do screening?
  • Screening is done on people who appear healthy, to detect disease early so that early treatment, better treatment, better prognosis, long & prolonged survival can be achieved
  • This advantage of screening is known as lead time

Lead Time

Without using screening tests, when will the disease be diagnosed later and by using screening test, how much earlier a disease can be detected in early stages, i.e., leading the time for detection of diseases. It is an important advantage of screening.

Survival Analysis 

• Upper curve: With treatment
• Lower curve: Without treatment
• 5-year survival rate is measured from people with and without treatment called as Kaplan Meir analysis 
• Cancer with highest 5-year survival post-screening is colonic cancer.
• It has >90-95% survival if its precancerous stage FAP, (Familial Adenomatous Polyposis) is detected early & treatment is given.

NHP’s	Screening	Diagnosis
1. TB	Any patient with cough > 2 weeks	Diagnostic test (India): Sputum smear by ZN stain Other tests:  1. CBNAAT (cartridge-based nucleic acid amplification test) 2. Automated liquid culture  3. Line Probe Assay 4. CXR 5. Tuberculin Test
2. Malaria	Any case of fever	PBF with JSB stain Rapid diagnostic kit
3. Leprosy	Any skin patch with Hypo-anesthesia	Clinical examination no. of skin lesions & nerve involvement is noted
4. HIV	E/R/S	Western blot assay (WBA)
5. DM	RBS	FBS > 126 mg/dl OGTT > 200 mg% HbA1C > 6.5 %
6. Cervical Cancer	VIA (visual inspection of the cervix with 5% acetic acid) > PAP Smear	Colposcopy Punch Biopsy
7. Breast Cancer	Mammography, MRI (young)	FNAC, biopsy

Properties of Screening Test

	Disease Present	Disease Absent
ST +ve	TP (a)	FP (b)
ST -ve	FN (c)	TN (d)
-	Cases: a + c	Controls: b + d

• Sensitivity = a/a+c *100 
• Specificity = d/b+d *100
• PPV = a/a+b *100 
• NPV = d/c+d *100 
• Positive out of the disease: Sensitivity
• Diseased out of positives: PPV
• Most useful property: Test with highest sensitivity

Important Information

• Diagnostic power of screening test is given by: PPV
• Diagnostic power of screening test to correctly exclude the disease is given by: NPV

Bayes Theorem         

• Sensitivity, Specificity & Prevalence are given. Then PPV is
  • PPV = Sensitivity Prevalence /Sensitivity × Prevalence + [1-Prevalence] *100  
• For Simplicity/MCQ (use all values in percentage)
• In formula write 100 below 1
  • PPV = Sensitivity *Prevalence /Sensitivity x Prevalence+[1100-specificity] [1100-prevalence] *100  
• Prevalence Aka pretest probability of a disease
• PPV: Aka post-test probability of the same disease in a population

Also Read: NEET PG 2023: High-Yield Topics For PSM

Screening Tests in Series (In parallel all these outcomes will be opposite)

• Sensitivity: Decreases
• Specificity: Increases
• PPV: Increases
• NPV: Decreases

PRECISION V/S ACCURACY

• Precision: Repeatability/ Consistency/ Reproducibility/ Reliability of test
• Measured by two charts 
  • R chart 
  • Range chart
• Accuracy: Whether the test is correct or not; How close is it to actual/ true value. Now called validity.
• Measured by 
  • Mean chart
  • Shewhart control chart
  • LJ chart

Receiver Operating Characteristic (ROC) curve

• Trading off between sensitivity & specificity is achieved by the ROC curve. 
• Z line: No use: Discard the test
• Y line: Fair
• X line: Good
• Uppermost line is taken as a cutoff for detection of a disease in a population.
• Upper the line, better the test

Previous Year Questions

Q. Best breast cancer prevention level is? (NEET 2019)

A. Specific protection

B. Early diagnosis and treatment

C. Disability limitation 

D. Rehabilitation

Q. Probability that a person out of those who tested positive on a screening test has in fact, the disease is known as? (NEET 2019)

A. Sensitivity

B. Specificity

C. Positive predictive value

D. Negative predictive value

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