1. School of Public Health, Faculty of Health Sciences, Univ. of the Witwatersrand. Johannesburg 2193, ZA.
2. Pathology Division, National Institute for Occupational Health, National Health Laboratory Service, Johannesburg 2001, ZA.
3. Council for Scientific and Industrial Research, Pretoria 0001, ZA.
4. Department of Biomedical Technology, Faculty of Health Sciences, Univ. of Johannesburg, Johannesburg 2006, ZA.