[1] Univ Stellenbosch, Fac Med & Hlth Sci, Div Chem Pathol, Cape Town, South Africa
[2] Univ Cape Town, Fac Med & Hlth Sci, Div Chem Pathol, Cape Town, South Africa
[3] Univ Witwatersrand, Fac Med & Hlth Sci, Div Chem Pathol, Johannesburg, South Africa
[4] Univ KwaZulu Natal, Fac Med & Hlth Sci, Div Chem Pathol, Durban, South Africa
[5] South African Med Res Council, Noncommunicable Dis Res Unit, Cape Town, South Africa
[6] Natl Hlth Lab Serv, Johannesburg, South Africa