Contrast-induced acute kidney injury (CI-AKI), previously known as contrast-induced nephropathy (CIN), is a syndrome in which an acute renal dysfunction is diagnosed after the intravascular injection of contrast media. AKI implies an injury or damage but not necessarily a reduction in overall renal filtration function. The renal damage becomes evident only when more than 50 % of the renal mass is compromised. This typically occurs when AKI is diagnosed using creatinine as a marker; in fact, creatinine is a surrogate of glomerular filtration and it does not describe the whole spectrum of kidney function. Recent AKI classifications include even slight changes in serum creatinine (as low as 0.3 mg/dl), which are associated with worse outcomes. An early diagnosis of AKI using novel biomarkers has now become possible. These new biomarkers provide additional value, not only because they facilitate earlier diagnosis but also because they can diagnose AKI even in the absence of a change in subsequent filtration function. Thus, in this situation, these new criteria can reveal subclinical AKI. A new domain of AKI diagnosis could then include functional and structural criteria as indicated by laboratory testing.