This cross-sectional study assessed the PPV of a clinical score for viral failure among patients on first-line ART who fulfilled the immunological and/or clinical WHO-criteria for treatment failure in rural Lesotho. The score applied in this study was originally derived from a cohort in Cambodia with 764 individuals 
. We found that a score≥5 had a high PPV for viral failure within a subgroup of patients in rural Lesotho who already fulfilled the immunological and/or clinical WHO-criteria. The PPV was 100% for a detectable viral load and 90% for a VL≥5’000 copies, while for the WHO-criteria alone the PPV was 71% and 51%, respectively (). Moreover, patients with a score≥5 tend to have a higher mortality while waiting for, or shortly after, the start of second-line ART. Because of the operational challenges to obtain a viral load in our setting (as highlighted in ), it may be appropriate to switch patients who fulfill the immunological and/or clinical WHO-criteria and have a score≥5 immediately to a second-line regimen without doing a confirmatory VL test. However, given that even those with a score<5 have a reasonable chance of failure (40%) the score cannot be used to exclude viral failure in this subgroup of patients.
This study has several limitations. First, we only measured one viral load per patient. A paper by Castelnuovo et al. showed that a majority of patients who have intermediate or high viremia achieve viral suppression at a second measurement 
. Saag and colleagues report from a trial in Zambia that nearly half of patients achieved viral suppression at second measurement 
. The overall failure rate may thus be overestimated in our study. A second limitation is that the applied score still relies on CD4-counts and is therefore not applicable in settings without access to CD4-count measurement. However, Marinucci et al. showed in an assessment in seven African countries that by 2010 already two thirds of rural ART-clinics had access to CD4-count on site 
. Like the health centers in Lesotho, many other clinics that do not have CD4-counters on site may have a system of regular transport of blood samples to the referral hospital for analysis. A third limitation is that the patient population included may not be fully representative of all patients possibly failing first-line therapy. The study suffers from operational challenges inherent to working in low-resource rural settings. Only 92 (69%) out of 134 eligible patients could be included in the study, as for the others no VL-result could be obtained. Patients have to travel long distances to the health facilities and may be reluctant to come back for a blood test, especially when sick. This demonstrates the need to define criteria that allow switching patients with high probability of viral failure to second-line without prior confirmation through VL testing. Finally, in this rural setting some patients may not have a regularly documented CD4-count while on ART and ART-nurses may have overlooked some clinical WHO stage 4 conditions. Thus the overall number of patients fulfilling clinical and/or immunological WHO failure criteria in the study area may be considerably higher than the 134 patients that were identified during the study period.
Future studies on scoring systems in low-resource settings should look at criteria that do not depend on previous CD4 counts. Our study only looked at the PPV of the score≥5 in addition to WHO criteria. Looking at the individual PPV of the criteria that build the score, the strongest predictors are VAS<95%, a new onset papular pruritic eruption and a CD4-count <100 cells/µl after ≥12 months on ART (). As in some settings it may be difficult to have all data available to use the score (previous CD4-counts and previous hemoglobin levels), using these criteria alone as additional check before switching patients who fulfill immunological or clinical WHO-criteria to second-line would already increase the PPV for viral failure substantially.
The CD4-cell count alone, as it is used in the WHO definition of immunological failure, has shown limited accuracy in predicting viral failure 
and has led to misclassification and unnecessary as well as delayed switch to second-line ART 
. Because of the limited accuracy of WHO criteria, confirmation of treatment failure through VL testing has been recommended for all patients 
. In our study, the PPV of WHO immunological criteria was higher than in previous studies 
, but a PPV of 44% for a VL≥5000 copies () is still too low to justify switch to second-line ART without prior confirmation by VL-testing. However, to await a VL result for all patients presenting with failure is difficult in rural settings with limited resources and may result in delayed switching and high mortality. Several studies indicate that use of other laboratory and clinical information, such as adherence or changes in hemoglobin may improve the predictive value 
. In a South African cohort, Egger and colleagues have recently shown the usefulness of risk-charts that take into account CD4-trajectories over time on ART 
. However, up to now, none has shown enough accuracy to replace VL testing. As long as cheap and simple to use tests for VL measurements are lacking, there is an urgent need to define clinical criteria that help to narrow down the population that needs VL testing.
A clinical score≥5 in patients fulfilling the clinical and/or immunological WHO treatment failure criteria results in a high probability of viral failure. For these patients, especially in settings with limited access to VL-testing, we may consider switching patients to second-line treatment without prior VL-confirmation. This approach may save money and valuable time for the patient, thereby reducing mortality and lost-to follow-up. However, a lower score cannot be used to exclude viral failure. These patients require a confirmatory VL measurement prior to switch to second-line.