Chronic kidney disease of uncertain etiology (CKDu) is a term coined to describe endemic chronic kidney disease (CKD) that has not been found or associated with its conventional predominant risk factors such as hypertension and diabetes1,2. This novel form of CKD is a major health issue that has gained public attention due to its high mortality3,4 in several tropical regions including Sri Lanka, India, Latin America and Egypt5–8. As this health condition mainly afflicts young and middle-aged males from the low socioeconomic agricultural community1,9, hypotheses to etiologies of CKDu have been focused on occupational triggers including environmental toxins that might have leached from the use of pesticides and aluminium vessels to drinking water, and ingested by these farmers. Despite many investigations on contaminations of arsenic, cadmium, lead, fluoride, chromium, aluminium and agrochemicals in the geographical resources10–15, causative factor/s of CKDu remains inconclusive. This lack of understanding in the etiology of CKDu has therefore garnered many relevant studies to seek a holistic comprehension of this disease.
CKDu has been observed to be a distinct form within the categorical diagnosis of CKD in regard to both clinical and pathological presentations. Whilst majority of CKD patients are presented with high serum creatinine and proteinuria, the latter is minimal in CKDu patients especially in early stage1. This is probably attributed to the fact that CKD suffers predominantly from glomerular and vascular damage16 whereas CKDu is hallmarked with the pathology of tubular interstitial compartment of the kidney17–19. Thus, evaluation of tubulointerstitial lesions in CKDu remains as the current “gold standard” for diagnosing as well classifying the severity17. Most of the current treatment for renal diseases are based on glomerular pathologies hence the serum creatinine and proteinuria-based approaches are appropriate. In contrast, renal tubules are neglected, at least in to a certain extent, beyond its involvement in diverse array of functions to maintain the body physiology. More specific, reliable and sensitive markers are required to evaluate the specific functions and severity of tubular interstitial diseases, hence, the CKDu.
Tubular markers as indicators for CKDu are highly plausible as the disease has been characterized with renal tubular damage as first and early sign20. Indeed, urinary alpha-1-microglobulin (A1M), a marker of proximal tubular dysfunction is probably the best single biomarker to identify CKDu/CKD at earliest stages. A1M reported to be elevated in CKDu, in comparison to healthy controls and the quantity, increases as the disease progresses20,21. There were several studies investigating the effectiveness of different tubular markers, kidney injury molecule 1 (KIM1) and neutrophil gelatinase-associated lipocalin (NGAL) for screening and diagnosis of CKDu22,23,24. Interestingly, one study reported elevated urinary KIM1 and NGAL was found not only in CKDu but also in healthy cohorts23. Findings strongly suggest subclinical kidney disease unless otherwise proven by a kidney biopsy or by follow up. Fernando et al, in 2019 proposed that using biomarker combination has better performance than using single marker for distinguishing CKD/CKDu and healthy controls21. This study demonstrated that a combination of urinary A1M, KIM 1, and RBP4 as a marker panel for distinguishing CKD/ CKDu from healthy controls, while a panel consisting urinary Osteopontin (OPN), KIM1, and RBP4 differentiating CKDu from CKD with high sensitivity and specificity21.
A preliminary data from multiplexed immunoassay motivated us to explore the effectiveness of serum retinol binding protein 4 (RBP4) in the evaluation the tubular functions, more specifically the proximal tubular functions of CKDu. We propose the concomitant use of glomerular and tubular markers would provide more realistic picture of chronic kidney disease opening new avenues for treatment. RBP4 is a small protein of 21kDa that is freely filtered through the glomeruli upon releasing of retinol. It is then readily reabsorbed by the proximal tubule and catabolized25,26. As such, it has been regarded as an excellent marker candidate for tubular renal impairment27,28. Moreover, there is the possibility that leakage of RBP4 through tubular damage, and thereby, increase in urinary RBP4 can be observed before the sight of glomerular markers28. This would be extremely relevant in the context of CKDu. Some of early studies on CKD reported significant positive correlation of serum RBP4 with serum creatinine29,30,31. Xun et al (2018) showed that serum RBP4 is useful in diagnosis of CKD as the concentration of serum RBP4 has an association with renal functions29. They confirmed that combined detection of serum RBP4 and routine renal function biomarkers such as serum creatinine, urea, cystatin C would improve diagnostic accuracy for CKD29.
In the same line, concerted efforts have been made to assess the applicability of current CKD diagnostic tools for CKDu diagnostics. For example, the urine dipstick test, a cost-effective, sensitive screening test in CKD, proven to have a limited role in minimally proteinuric early stages of CKDu32. Further, eestimated GFR (eGFR) has been regarded as the most popular indicator of kidney dysfunction and this index takes into account variables including S.Cr, age, weight, and gender33. Though, eGFR (< 60 mL/min/1.73 m2) is an essential criterion in the diagnosis of CKDu, there is a strong possibility for the existence of significant kidney damage, even among so called “healthy” individuals with eGFR more than 60 mL/min/1.73 m2 34. This indicates the unknown bona fide impact in the diagnosis of CKDu, a primary tubular interstitial disease, has on eGFR. Hence, RBP4 a tubular marker, in combination with Creatinine or eGFR is complementary in the evaluation of severity of tubular interstitial diseases.