On behalf of all the authors, we would like to convey our gratitude to you for showing interest in our publication, as reflected in your letter to the editor1,2.

The sample size is indeed a limitation of our work and this is reflected in the discussion, even though this is the largest Spanish prospective series to date of acute kidney injury (AKI) in patients with COVID-19 directly treated by nephrology. This implies a negative selection bias for the most critical patients who required referral to the nephrologist due to the severity of their condition, the need for dialysis or additional complications. Under no circumstances can we take our series of 41 patients as representative of AKI in the COVID-19 patient in general. In fact, our analysis approach is fundamentally descriptive. Our focus was on highlighting the tremendous variability of causes, clinical presentations and outcomes we have collected and which go beyond direct kidney involvement due to the SARS-CoV-2 infection or resulting from the cytokine storm. We wanted to give a perspective from the point of view of the nephrologist, providing clarity at a time when there were large gaps in our knowledge. If you remember, the Ministry's COVID-19 report of April 2020 did not acknowledge chronic kidney disease (CKD) as a risk factor, and nor did it provide published evidence that AKI was a problem associated with COVID-193.

As for the differential diagnosis between AKI and CKD, we specified in the methods section that the KDIGO criteria were used to diagnose AKI. It is then clarified in the section on baseline characteristics, within the results, that 36.6% of our population had underlying CKD. In our previous analysis of 1.600 patients admitted to the hospital as a whole during the first wave, the association of CKD and AKI was specifically analysed separately in the prognosis of the patients4. In the same issue, editorial comments precisely stress this point, the difficulty of separating pure AKI from the deterioration of pre-existing CKD5.