Long-term effects of COVID-19 on kidney function - Authors' reply.

In our study, we observed a considerable number of patients presenting with low estimated glomerular filtration rate (eGFR) over time without acute kidney injury and an eGFR of 90 mL/min per 1·73 m2 or more during hospitalisation. Another study has shown that the use of creatinine to diagnose acute kidney injury might underestimate the patients with acute kidney injury at acute phase. We found that reduced eGFR at follow-up is possibly associated with kidney injury at acute phase, which was not recognised on the basis of serum creatinine values.

Philipp Enghard and colleagues reported that other factors (eg, hydration) could lead to fluctuations of serum creatine values and further affect the calculated eGFR values. Patients categorised into the group with an eGFR of 90 mL/min per 1·73 m2 or more at acute phase all had eGFR values that were 90 mL/min per 1·73 m2 or more during hospitalisation. According to our data, 1366 (80·1%) of 1703 had at least two eGFR values and 956 (56·0%) of 1703 patients had at least three eGFR values, which, to some extent, excluded the possibility of misclassification resulting from fluctuations. For the eGFR value at 6 months after symptom onset, fluctuations might have existed, as the value was obtained once. However, attention should be paid to the group of people that had a lower eGFR value after discharge than at acute phase and are at risk of long-term kidney damage, and who need help from health-care providers and further follow-up to differentiate kidney damage from fluctuation or physiological decrease.

Pierre Delanaye and colleagues reported that among patients without acute kidney injury during acute phase, an eGFR less than 90 mL/min per 1·73 m2 during follow-up might be attributable to lowered eGFR before COVID-19. For patients without acute kidney injury and with an eGFR less than 90 mL/min per 1·73 m2 at acute phase, the possibility of an eGFR less than 90 mL/min per 1·73 m2 before COVID-19 cannot be excluded. However, for those patients without acute kidney injury and an eGFR of 90 mL/min per 1·73 m2 or more at acute phase, an eGFR less than 90 mL/min per 1·73 m2 should be paid attention to because the possibility of an eGFR less than 90 mL/min per 1·73 m2 before COVID-19 is quite low. We agree that the criteria of an eGFR less than 90 mL/min per 1·73 m2 cannot define chronic kidney disease, especially for those with an eGFR between 60 and 90 mL/min per 1·73 m2 in the absence of proteinuria; although, we do not want to ignore patients with preclinical manifestation of kidney damage as proteinuria was not measured then. We hope these important questions can be further answered in future studies.

Kidney involvement in patients with COVID-19 is critically important and more attention should be paid to renal consequences after COVID-19 because acute kidney injury could result in short and long consequences in adults and children. Patients without acute kidney injury at acute phase are also at a potential risk of kidney function deterioration over time, which needs to be validated in future follow-up studies and further investigated for the potential pathogenesis.