081029L -CASE ANALYSIS: JENNY AND LI

Material by Professor Omar Hasan Kasule Sr. for Year 2 Semester 1 Med PPSD session on 29^th October 2008

Background

An increasing number of liver conditions forced the General Hospital to set up a separate liver clinic. A decision had to be made on referral of patients to the clinic using a simple and cheap screening test. It was decided to try simple tests based on serum enzyme assay: aminotransferase    to detect hepatocellular damage and alkaline phosphatase to detect biliary obstructions. A 6-fold increase of aminotransferase was set as indicating hepatocellular damage and a 2.5-fold increase of alkaline phosphatase was used to indicate biliary obstruction. The physicians were aware that these tests would not always be accurate and before staring their pilot project decided to study the performance of the 2 tests. After obtaining informed consent, they administered the tests to a group of 200 patients who presented in the general outpatient clinic with complaints relating to liver disease. Then confirmatory tests were carried out. Hepatocellular damage was assessed by liver biopsy and histological examination. Biliary obstruction was assessed radiologically. The following tables show the results of the screening and the confirmatory tests.

Table 1: Screening for hepatocellular damage

	Hapatocellular damage +ve	Hepatocellular damage -ve
Aminotransferase > x 6 times	80	10
Aminotransferase < x 6 times	20	90

Table 2: Screening for biliary obstruction

	Biliary obstruction +ve	Biliary obstruction -ve
Alkaline phosphates > x 2.5  times	90	80
Aminotransferase < x 2.5 times	10	20

Discussion:

1. Compute the sensitivity and specificity of the screening test..
2. Compute the false positive and the false negative proportions for the screening test.
3. Compute the positive predictive value (PV+) and the negative predictive value (PV-) for the test