4.2 Methods
4.2.4 Sampling strategy
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4.2.4.2 Comparative survey
The sampling frame included all IMCI trained health workers working in primary health care clinics in KZN and Limpopo provinces at the time of data collection; all eligible health workers were nurses.
Sample size for observed consultations
The number of consultations to be observed for each health worker, to effectively assess competence of health workers in identifying and managing children with HIV infection, was determined by the prevalence of SUSPECTED SYMPTOMATIC HIV in the clinic population. In order to assess health worker performance, each observed health worker needed to assess and classify at least one child with this classification. However, HIV prevalence among children in communities or in the clinic population was unknown, and the only data available were the HIV prevalence among women attending government antenatal care services. The antenatal HIV prevalence for each province was used to estimate the prevalence of HIV infection in children under five years. In KZN, the antenatal prevalence was 40.7% in 2004 [37]. Assuming that one-third of children become HIV infected through vertical transmission, the community prevalence in children under five years could be estimated to be around 13.5%. In Limpopo, similar assumptions suggested a community prevalence of approximately 6.5%. However, this may have been an overestimate since many HIV infected children die in the first year of life, and HIV infected children are likely to attend health facilities more frequently. Using these estimates, while acknowledging that they are imprecise, we planned to observe each health worker for 20 consultations. Thus, in Limpopo, observed health workers were expected to see one child and, in KZN, two children with SUSPECTED SYMPTOMATIC HIV during the 20 consultations observed.
Sample size for observed health workers
The sample size calculation was based on the determination of two outcomes, both related to the health worker’s performance in implementing the HIV component of IMCI.
The first outcome was the sensitivity of the HIV algorithm when used by IMCI trained health workers compared to its use by an expert IMCI practitioner. This was calculated by estimating the proportion of HIV exposed or infected children correctly assessed and classified for SUSPECTED SYMPTOMATIC HIV by an IMCI trained health worker as compared to an IMCI expert (primary outcome 5a).
The following calculation was to determine the number of HIV positive children required in order to estimate the sensitivity of the algorithm.
The sample size calculation was based on the assumption that 80% (+/- 10%) of children with SUSPECTED SYMPTOMATIC HIV would be correctly identified by observed health workers, with
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acceptable confidence limits of sensitivity between 70% and 90%. If the number of children examined by each health worker was 20, it was assumed that one of these children would have SUSPECTED SYMPTOMATIC HIV. The sample size was calculated using the formula for comparing two proportions:
P1 (1-p1) + P2 (1-p2)
n = --- (Z + Z)2
(P1 – p2)2
Thus, the number of symptomatic HIV cases required: = (1.96)2 (0.8) (0.2) = 62 (0.1)2
The number of children with SUSPECTED SYMPTOMATIC HIV required for observation was 62 and, based on the assumption that each health worker would be observed assessing at least one child with SUSPECTED SYMPTOMATIC HIV, the total number of health workers to be observed was 62.
The second outcome was the proportion of health workers who would make a correct classification for HIV in all 20 cases that they examine. This is a health worker based outcome and was calculated by estimating the proportion of all health workers observed who would classify all children correctly as having SUSPECTED SYMPTOMATIC HIV or not, as compared to an expert IMCI practitioner (primary outcome 5b). The sample size was calculated by assuming that the
proportion of health workers able to correctly classify for HIV in all the children examined would be 80% with acceptable limits of the expected proportion between 70% and 90%.
Number of health workers required was therefore: = (1.96)2 (0.8) (0.2) = 62 (0.1)2
The number of health workers to be observed was, therefore, 62 and sampling was stratified by province with equal numbers selected in each province (31 health workers in each of the two provinces). This calculation was not dependent on the prevalence of SUSPECTED SYMPTOMATIC HIV in the observed population. Participants were randomly selected from a list of all IMCI trained health workers in each province obtained from the provincial IMCI co-ordinator. Selected health workers were not informed ahead of time that they were to be observed, but a general circular was sent out by the provincial Child Health Programme Manager informing staff that a survey of child health practices was to be undertaken and that observers would be visiting clinics to undertake observations of health workers managing sick children.
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Determination of HIV prevalence among clinic attenders
From these calculations, 600 consultations in total were to be observed in each province. HIV test results from these children would be used to determine the prevalence of HIV infection among clinic attenders. The sample size required to determine accurately HIV prevalence in this population was calculated as follows:
P(1-P) n = z2 ---
2
Z= 1.96; P= prevalence; n= sample and is absolute precision
Thus, with a sample of 600 observed children in each province, the precision of 95%
confidence intervals at different prevalence rates from 5 to 15% is shown below:
Prevalence n Precision Lower Limit Upper Limit
5% 600 +1.7% 3.3% 6.7%
7% 600 +2.0 5.0% 9.0%
10% 600 +2.4 7.6% 12.4%
15% 600 +2.9% 12.1% 17.9%
However, as the study progressed and in the light of an interim analysis, the sample size was revised to ensure that an adequate number of assessments were included to meet the primary objectives. The interim analysis showed that only 26% of health workers had correctly classified all children, and the community prevalence of HIV in Limpopo province was only 2%. The sample was, therefore, recalculated, and increased to 77 health workers. An additional 15 IMCI trained health workers were randomly selected in KZN, where HIV prevalance was higher.