An atypical case of acute kidney injury: Answers

Abstract

Hemolytic uremic syndrome (HUS) is characterized bymicroangiopathic anemia, thrombocytopenia, and acute kidney injury [1]. In addition, severe HUS can be associated with irreversible renal and CNS complications, leading to chronic kidney disease and permanent neurological deficits [2]. The most common form of HUS is induced by a verocytotoxin/ Shiga toxin (Vtx/Stx)-producing bacteria [3]. Less commonly, HUS can be induced by bacterial (e.g., S. pneumoniae) [3] or viral (e.g., influenza) infections [4]. We report a patient with a severe clinical presentation of HUS following pneumonia (presumed S. pneumoniae infection). Making the correct diagnosis proved to be difficult as the patient presented with unusual findings, including a negative Coombs test and low C3 complement level on two occasions. These findings did not meet the usual diagnostic criteria of S. pneumoniae-induced HUS, which include a positive Coombs test and a normal complement C3 level. While HUS caused by defective complement alternative pathway (CAP) regulation, presents with low serum C3 complement in approximately 75 % of patients [5], patients with the infection-induced types of HUS do not exhibit complement dysregulation, and their C3 levels are normal [3, 6]. In patients with S. pneumoniae-induced HUS, the C3 levels are usually normal, and the Coombs test is positive, owing to antibodies binding to the exposed T-antigen on the surface of red blood cells [7, 8]. Thus, the combination of normal C3 complement levels with a positive Coombs test would confirm the diagnosis of S. pneumoniae-induced HUS [7, 8]. However, complement activation leading to a lower C3 complement can occur even in non-genetic forms of HUS [9], which can be considered an aggravating factor for the disease progression. Furthermore, the Coombs test can be positive in 25 to 90 % of cases [10, 11], indicating that a relatively large proportion of patients with pneumococcal HUS can have a negative Coombs test. This refers to the article that can be found at http://dx.doi.org/10.1007/ s00467-015-3101-y