This study aimed to evaluate urinary concentrations of three LMWP in relation to total urinary protein and albumin in a multiethnic HIV cohort and study potential associations of these parameters with the type of cART used. We observed poor correlations between LMWP and PCR or ACR, and no significant overall differences in urine PCR, ACR and LMWP excretion between patients stratified by cART and TFV exposure. RBPCR in 95% of patients was within the reference range. By contrast, 67% of patients had elevated NGALCR measurements.Black ethnicity was associated with reduced urinary concentrations of LMWP, and impaired renal function with increased urinary RBP concentrations.
Urinary RBP and cystatin C have previously been proposed as biomarkers for monitoring tubular function in HIV positive patients receiving TFV [25, 33]. In a recent clinical trial, a 50% increase in urinary RBP excretion was observed in patients who initiated TFV with an NNRTI, with no change in the abacavir arm . In a previous cross-sectional study, Hall and colleagues reported increased urinary RBP levels in patients receiving cART including TFV, compared to patients receiving cART without tenofovir or cART naive patients . Our study differed from the latter study in that it included a 3-fold larger cohort, further stratified patients by TFV/NNRTI and TFV/PI exposure, and conducted a multivariate analysis of factors associated with UQ RBPCR measurements.
The absence of a significant association between raised RBPCR and TFV exposure in general, or TFV/PI exposure more specifically, may relate to the fact that our patients had no evidence of clinical renal tubular toxicity, the size and heterogeneity of the cohort, the assay or the RBPCR cut-off chosen for our analyses, or unmeasured confounding. Of note, the median RBPCR in a previous study of patients with TFV-induced Fanconi syndrome was 5593 μg/mmol (49,515 μg/g) , which is approximately 100-fold higher than the upper limit of the reference range of the assay used in our study. As almost all of our patients had RBPCR measurements within (or slightly above) the reference range, RBPCR appears to have good discriminatory value between severe, treatment-limiting renal tubular disease and normal tubular function or mild, asymptomatic renal tubular dysfunction.
HIVAN is the predominant cause of end-stage kidney disease in black patients with HIV infection [3, 4, 11, 12]. In more advanced cases of HIVAN, urinary NGAL concentrations are typically elevated, and NGAL has been put forward as a useful urinary biomarker to aid the diagnosis of HIVAN [37, 38]. Given the strong ethnic predisposition of HIVAN and that this condition may be present in patients with preserved renal function and mild proteinuria , it was surprising to find black ethnicity to be associated with lower odds of UQ NGALCR. However, as HIVAN affects only a small proportion of black patients in the UK , it is possible that increased urinary NGAL levels in those with (subclinical) HIVAN were masked by generally lower NGAL levels in black patients without HIVAN.
Our results suggest that RBPCR results may need to be interpreted in the context of eGFR and ethnicity. The observed association between ethnicity and tubular biomarkers is of interest and suggests that tubular handling of LMWP may be different in patients of different ethnic backgrounds, or that the prevalence of genetic polymorphisms of the organic anion transporters and/or multi-drug resistant proteins, which have been implicated in the pathogenesis of cART-associated tubular dysfunction [27, 40], may vary by ethnicity. Alternatively, a higher muscle mass in black patients may have resulted in higher urinary creatinine concentrations, and thus somewhat lower RBPCR, NGALCR and CCR values.
In our cohort, NGALCR poorly correlated with either RBPCR or CCR. Although all these LMWP are generally proposed as biomarkers of proximal tubular dysfunction, the lack of correlation may suggest that urinary NGAL and RBP or cystatin C reflect different types and/or severity of tubular dysfunction. NGAL is considered to be a sensitive, early marker of acute kidney injury (acute tubular necrosis), i.e. of a state of extensive global injury of proximal tubular cells . By contrast, RBP and cystatin C may be better markers of specific forms of proximal tubular dysfunction (such as inherited forms of Fanconi syndrome)  or states where proximal tubular dysfunction relates to mitochondrial toxicity – and thus a decrease in energy supply - and not to actual necrosis .
In the FRAM study, microalbuminuria was present in 11% of HIV positive patients, and associated with several cardiovascular risk factors including insulin resistance and hypertension, as well as immunodeficiency . Several studies in the general population suggest that urine albumin excretion at the level of microalbuminuria associates with increased risk of cardiovascular events and death, whereas protein excretion at the level of macroalbuminuria or clinical proteinuria (i.e. urine protein >0.5 g/day) predicts both cardiovascular events and CKD progression . The associations of elevated albumin and protein excretion with future risks of developing renal failure, cardiovascular events and death have also been confirmed in studies in HIV positive patients [44–47], and thus, urine albumin and protein excretion should be assessed and taken into account in cardiovascular and CKD risk reduction strategies.
This study has several limitations. It followed a cross-sectional design and, thus, longitudinal associations could not be evaluated. We relied on a single PCR, ACR and LMWP measurement for our analyses, and we lacked data on phosphate reabsorption or renal histology. Despite the overall sample size, the number of patients receiving TFV/PI was relatively small, and HIV negative controls were not included. Nonetheless, the ethnically diverse participants in our study are representative of the HIV population in the UK, and patients were only selected by their willingness to participate in the study and not for the presence or absence of specific clinical characteristics.