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dialysis center in the month preceding viral infection and during infection and found a critical difference (Figure 1). In the patients who tested positive for COVID-19, the mean (AESD) ferritin levels in March (before viral infection) and at diagnosis were 584 AE 318 and 1446 AE 1261 ng/ml, respectively, which was a mean increase of 275%. Interestingly, ferritin levels were increased at diagnosis in the 5 asymptomatic patients as well as in the patients with symptoms (mean AE SD, 1209 AE 1292 and 1535 AE 1280 ng/ml, respectively). Ferritin levels remained stable or decreased very slowly during the whole period of sickness in almost all patients. The pathophysiological mechanisms underlying high ferritin levels have not been totally explained at this time, and some investigators have reported a cytokine storm syndrome or macrophage activation syndrome; however, in our cohort, ferritin levels were not correlated with C-reactive protein (data not shown). 4,5 Screening for COVID-19 in hemodialysis centers is crucial so that infected patients can be isolated and to protect noninfected patients. Ferritin could be a helpful, available, and easy-to-use screening tool for the disease, although we believe that more research still is needed.

Primary renal lymphoma is a rare clinicopathologic entity that typically presents as renal mass or renal impairment with enlarged kidneys. We describe the case of a 66-year-old woman who presented with type II mixed cryoglobulinaemic vasculitis as the first manifestation of underlying low-grade primary renal lymphoma.

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Although dialysis patients are known to have impaired antibody responses to pathogens and fluctuation of antibody levels, 1 the response to coronavirus disease 2019 (COVID-19) in this population remains to be determined. De Vriese and Reynders 2 present the first evaluation of potential antibody responses in a dialysis population. We agree that 2 sequential negative COVID-19 swabs before de-isolating dialysis patients is a reasonable approach, as we recently demonstrated. 3 De Vriese and Reynders studied the presence of immunoglobulin G (IgG) against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid (N) protein in 7 patients and concluded that patients develop an antibody response within 15 days. Although we acknowledge that these findings are novel, we highlight that they might not be applicable to other dialysis populations with COVID-19 infection. First, the infection rate in their population was very low (7/289 [2.5%]) and most of these patients had a severe form of the disease (3 [43%] died and 1 was still in the intensive care unit [ICU]). In comparison, in our experience, 4 11.3% of our hemodialysis population had COVID-19 infection and only 7 of 76 (9.2%) died. IgG against SARS-CoV-2 N protein has been shown to be higher in ICU compared with non-ICU patients. 5 Therefore, it remains to be elucidated whether those results are applicable to any hemodialysis population and whether there is a prognostic role in assessing anti-spike (S) protein IgG in combination with IgG against SARS-CoV-2 N protein. 5 Moreover, it remains to be confirmed how long these antibodies will last and their clinical relevance in larger populations.