SCREENING PROCEDURES IN HUMAN MEDICINE EVALUATION OF RESULTS BY MULTIPLE CORRESPONDENCE ANALYSIS Jože Rovan1, Vilma Urbančič-Rovan2, Mira Slak2 1Faculty of Economics, Dept. of Statistics, University of Ljubljana 2University Medical Centre, Dept. of Endocrinology Ljubljana, Slovenia
Screening mass examination of the population to detect the existence of a particular disease. Dorland’s illustrated Medical Dictionary, 25th Ed, WB Saunders, 1974
Diabetes mellitus • chronic metabolic disorder • elevated blood sugar levels • incidence: 4 - 7 % of the population • Slovenia: ~ 80.000 diabetics (4% of the population)
Late complications of diabetes develop after 7-10 years of high blood sugar: • blood vessel disease - diabetic angiopathy • nerve disease - diabetic neuropathy: • impaired sensation (pain, temperature, vibration, light touch) • decreased muscle strength • disturbed function of the autonomic nerves
The diabetic foot a group of disorders of the foot, caused by late complications of diabetes: • poor blood supply (ischaemia) • disturbed nerve function • infection
Gangrene and amputation are among the most feared complications of diabetes mellitus: • 50% of all non-traumatic amputations are performed on diabetics • foot ulcer develops in 15% of the diabetics • very often, gangrene and amputation of one leg are followed by gangrene and amputation of the other.
Foot screening protocol In order to prevent gangrene and amputation, the patients at risk for such complications must be detected on time and treated properly. Foot screening protocol is a world-wide adopted set of simple and cheap diagnostic procedures that helps us identify the patients at risk.
Data acquisition • Out-Patient Diabetes Clinic, University Medical Centre Ljubljana • observation period: Nov. 96 - Nov. 98 • foot screening procedure: • 1275 patients • 50.8% women, 49.2% men • average age 63.63 years
Data acquisition • demographic data (ID, age, sex) • medical history (previous foot ulcer, amputation, various symptoms) • foot examination (various deformities, hard skin, ulcer, dry skin, redness, arterial pulses) • risk status classification (groups 1 - 4) • therapeutic measures (education, footwear prescription, referral to: foot clinic, angiologist, surgeon, pedicurist)
Data acquisition • 56 variables were analysed • all, except for age, were nominal, mostly dichotomous • age has been recoded to 3 age groups: • 1: under 51 years • 2: 51 - 70 • 3: more than 70 • altogether 117 categories
Multiple correspondence analysis (MCA) Correspondence analysis is a multivariate method for exploring categorical data. The primary goal of MCA is to transform numerical information into graphical displays (“maps”) and related numerical statistics. The position of the category-points in MCA maps is the basis for revealing the relationship among the investigated variables.
Maps and analysis When the cumulative percentage of inertia of the first two dimensions is relatively high (i.e. 58.51% in our case), then most of the profiles are well represented in a two-dimensional map (by their projections onto a plane).
Absent pulses of distal arteries Risk group 3 Referral to angiologist
Absent pulses of distal arteries Risk group 3 Referral to angiologist Referral to foot clinic Acute foot ulceration
Male Nail changes Foot deformity Female Referral to pedicurist Abundant callus
Conclusions (1) • As we have expected, for most of the variables under consideration, there was not much difference in the characteristics of the left and the right foot. • The patients with poor blood supply form a special group.
Conclusions (2) • Abundant callus, hallux valgus and toe nail deformities are more frequent in women than in men - possibly due to improper footwear. • The category-points representing the groups with an acute foot ulcer, loss of protective sensation, absent foot pulses, foot deformity, abundant callus and history of previous foot ulcer were close together, confirming the influence of the known risk factors on ulcer development.
Conclusions (3) • By MCA, we have confirmed most of the causal relationships in the development of foot pathology that are known from the literature. • In human medicine, we are often faced with the situations where categorical (nominal and ordinal) variables are predominant. Even more, some laboratory results, although physical readings, are essentially of ordinal nature. For that reason, we believe MCA can be a fruitful approach in the analysis of medical data.