Understanding Specificity in Screening Tests
Specificity measures a diagnostic test's ability to correctly identify individuals who do *not* have a specific disease or condition.
- It is defined as the proportion of true negative results among all individuals without the disease.
- Formula: Specificity = TN / (TN + FP), where TN = True Negatives, FP = False Positives.
Interpreting 90% Specificity
A screening test with 90% specificity means:
- When applied to a group of people who are all known *not* to have the disease, the test will correctly identify 90% of them as negative.
- This implies that 90% of the non-diseased individuals will receive a true negative result.
- Conversely, 10% of the non-diseased individuals would incorrectly test positive (a false positive result).
Conclusion
Therefore, 90% specificity indicates that 90% of non-diseased persons will yield a true negative result.
Important Questions from Biostatistics
What is the specificity of sputum microscopy in detection of Pulmonary Tuberculosis (PTB) as per the information given below?
PTB Total Sputum microscopy Present Absent Positive 270 20 290 Negative 30 180 210 Total 300 200 500 In a cohort of 500 women attending antenatal clinic, 70 % had ultrasonography (USG). This cohort was followed up at delivery. Of the women who had USG, 70 delivered low birth weight (LBW) babies; whereas of the women, who did not undergo USG, 50 delivered LBW babies. The incidence of LBW babies among women who had USG is:
In a case-control study, 300 women aged 20-45 years suffering from breast cancer were compared with age-matched 300 women without breast cancer. It was observed that 120 women among cases and 60 women among controls were obese. The odds ratio of developing breast cancer among obese women is:
What is the relative risk of developing pulmonary embolism in users of oral contraceptives as per the information given below?
Women using OCs Pulmonary embolism Total Yes No Yes 120 80 200 No 10 70 80 Total 130 150 280 Which one of the following tests should be applied to compare mean haemoglobin level of two groups of antenatal mothers?
