Calculation of sensitivity of primary screening

Variables for use with listed formulae:

		Final smear report
		Abnormal		Normal
		HSIL+	ASCUS/LSIL	Negative/Inadequate
Cervical smear report prior to QC check	Abnormal	a	b	c
	Normal	d	e	f

 

Sensitivity  of primary screening=	abnormals identified correctly before QC check _____________________________________ abnormals identified after QC check x100
Overall sensitivity  of primary screening=	a + b ____________ a+b+c+d     x  100
Sensitivity of primary screening for HS I L+ =	a+f ______ a+d x 100

Definition of sensitivity and specificity

Sensitivity: The sensitivity of the test is defined as the proportion of subjects with the disease correctly identified as positive out of all persons (tested) with the disease

Specificity: The specificity of a test is defined as the rate of correctly identified persons without disease in relation to all persons (tested) without disease

Target population : A group of persons sharing pre-defined characteristics eligible for a particular test or investigation or the object of a particular study.

Method of calculation of sensitivity  and specificity and false negative rate of  the cervical smear test (Pap test) using histology as the gold standard

Pap test	Outcome
	Positive	Negative
Positive	True positive Pap test (A)	False Positive Pap test (B)
Negative	False negative Pap test (C)	True positive Pap test (D)

Sensitivity=	A ____ A+C	True Positive _______________________ True Positive + False Negative
Specificity =	D ____ D+B	True Negative ___________ False Positive + True Negative

False negative rate = 1- sensitivity
False positive rate = 1- specificity

Method of calculation of Positive Predictive Value & Negative Predictive Value using Histology as the Gold Standard

Positive predictive value	A _____ A + B
Negative predictive value	D ____ C+D

The PPV of a Pap test reflects the accuracy with which an abnormal smear result predicts cervical neoplasia. The  NPV of a Pap test reflects the accuracy with which a negative smear result predicts absence of disease.
Predictive values depend on the prevalence of CIN & cervical cancer in the population. The predictive values will vary according to whether the population screened are at high risk or low risk of cervical cancer.