Use of monitored CD4 cell counts: predictions of the AIDS epidemic in Scotland: CD4 Collaborative Group.

OBJECTIVE

We describe the CD4 database of the Scottish Immunology Laboratories, and its uses and limitations for making short-term predictions of a CD4 cell count less than or equal to 200 x 10(6)/l (CD4(200)) and of adult AIDS cases in Scotland.

DESIGN

The date of the earlier of two consecutive samples (typically 3 months apart) both with CD4 cell counts less than or equal to 200 x 10(6)/l was taken to define when a patient had passed the CD4(200) threshold (referred to as a CD4(200) case). The CD4 database comprises HIV-1-seropositive adults in the four main risk groups [homosexual/bisexual (1), injecting drug users (IDU; 2), heterosexual contact (3), and undetermined (9)] from Scotland's three principal areas of population (Lothian, Tayside and Strathclyde) who have had a CD4 cell count of less than or equal to 500 x 10(6)/l.

SETTING

Three hospitals in Scotland, the Communicable Diseases (Scotland Unit) and the Medical Research Council Biostatistics Unit, Cambridge, UK.

PATIENTS,

PARTICIPANTS

The CD4 database at 31 December 1990 listed 813 patients (of whom 52% were IDU): 390 were CD4(200)/AIDS cases (of whom 44% were IDU) and 192 were AIDS cases (of whom 32% were IDU).

RESULTS

Individuals in risk groups 1, 2 and 3 were nearly equally represented among newly diagnosed HIV-1 infections in 1990. However, among patients with moderate immunodeficiency, IDU accounted for 50% of the total number. Co-incidence of first CD4 cell count with CD4(200) diagnosis was recorded for only 28% of IDU, but in over 50% of cases for each of the other exposure groups (57%). There was a highly significant decrease of around 80 x 10(6)/l per calendar-year-of-referral in first CD4 cell counts for patients on the CD4 database; and decreases of around 40 x 10(6)/lper decade of age at referral. Since 1988, median time from CD4(200) to AIDS diagnosis in Scotland has been approximately 2 years. Back-projection was applied to annual CD4(200)/AIDS diagnoses before 31 December 1990 and to AIDS diagnoses. From AIDS diagnoses, the central epidemic scenario underestimated past HIV-1-antibody-positive reports (up to the end of 1985). More dramatic underestimation was occasioned by back-projection from CD4(200)/AIDS diagnoses [319 inferred HIV infections compared with 445 HIV-1-antibody-positive reports to Communicable Diseases (Scotland) Unit].

CONCLUSIONS

First CD4 cell counts should complement new HIV-1 diagnoses. Past referrals for immunological monitoring were not uniform between risk groups in Scotland. Underascertainment of CD4(200) cases is a problem when CD4(200) cases are used as a basis for back-projection. More information concerning the incubation distribution from HIV seroconversion to CD4(200) diagnosis is required. It is likely that there are twice as many CD4(200)/AIDS as there are diagnosed cases of AIDS.