Thrombotic microangiopathy (TMA) is a severe and life-threatening condition, often affecting the kidneys and brain. It can occur on the background of various clinical conditions. Dysregulation of the alternative pathway of complement may be the etiological factor and this type of TMA is classified, according to the current nomenclature, as primary atypical hemolytic uremic syndrome (HUS). Half the patients with primary atypical HUS present with rare variants in complement genes, although coexisting conditions are often needed for the TMA to become manifest. In patients with secondary atypical HUS, certain coexisting conditions appear to drive the disease and treatment should target the underlying condition to remit the TMA. Recently, the investigators demonstrated, by using a novel in-house developed functional endothelial cell-based test, that complement dysregulation and overactivation is the dominant cause of disease and its sequelae in a subset of patients with secondary atypical HUS, having impact on treatment and prognosis. The investigators did first prove this concept in patients presenting with TMA and hypertensive emergency. A prospective study is needed to further corroborate these findings along the spectrum of TMA. The investigators hypothesize that their functional endothelial cell-based test, the so-called HMEC test, can better categorizes the TMA into different groups with potential therapeutic and prognostic implications. Thus, paving the road to the ultimate goal of precision medicine.