The present study included 203 older adult patients. Among them, 36 patients were selected for the analysis based on the specified inclusion and exclusion criteria. These selected patients had a mean age of 71.67 years and a predominance of female patients in the study sample.
According to the Brazilian Society of Nephrology (SBN)22, the normal serum creatinine level range is broad, between 0.6 and 1.3 mg/dL and between 0.8 and 1.2 mg/dL according to Stevens et al 23. Additionally, serum creatinine values are influenced by variables such as age, sex, nutritional status, and muscle mass. Therefore, relying solely on plasma creatinine is not ideal for evaluating renal function, since elevated values above 1.3 mg/dL indicate a decrease of approximately 50–60% in GFR. In this regard, eGFR is the preferred measure, as it demonstrates an inverse relationship with creatinine 1,22-25. A rise in serum creatinine can serve as an initial indicator of potential kidney dysfunction.24 Nonetheless, it has been debated that using normal reference ranges based on older patients is not particularly effective in the early detection of kidney impairment.24
Supporting this fact, older individuals, especially women, commonly experience lower muscle mass, rendering the evaluation of renal function through GFR more relevant.3,5,6,8 Relying solely on creatinine values may lead to a misclassification of these patients as falling within the “normal range”, even in the presence of decreased GFR. As observed in the present study, the mean initial serum creatinine value was 0.87 mg/dL for all patients upon admission to the ICU, suggesting normal renal function. 6,8, 22, 23
However, this finding contrasts with eGFR values < 60 mL/min/1.73 m² upon ICU admission obtained using BIS-creatinine for 33.3% of the patients and using CKD-EPI for 18.2% of the patients. GFR values < 60 mL/min/1.73 m² indicated a reduction in the GFR and a manifestation of renal parenchymal injury.25 When this reduction persists for more than 3 months, it meets the diagnostic criteria for CKD according to KDIGO 25-28. Therefore, in this study, the values were categorized into GFR < 60 mL/min/1.73 m² and ≥ 60 mL/min/1.73 m².
As observed by Swedko et al. 29, serum creatinine is an unsatisfactory screening test for evaluating renal injury in older adult patients, resulting in a significant underinvestigation and underrecognition of renal insufficiency in this population 3-7,16, 24. The results of this study were in accordance with these findings. According to Swedko et al. 29, older adult patients may have undetected CKD masked by “normal” serum creatinine values, which can be misclassified as having normal renal function 5,6,8, 29.
However, correct identification of this comorbidity is important for providing more comprehensive healthcare during follow-up and ICU admission, since patients with CKD who develop AKI may have worse outcomes than those without this comorbidity 3,6,29.
Nascimento et al. 9 reported that AKI patients aged > 60 years who were referred late to a nephrologist were associated with higher mortality. Furthermore, Nascimento et al. 9, in a retrospective observational study, observed that delayed consultation with a nephrologist is associated with an increased risk of death, even after adjustments are made (OR 2.66, 95% CI: 1.36-4.35, p = 0.001) 9.
This finding has been corroborated by Swedko et al. 29, who stated that a lack of proper investigation and referral of older adult patients with renal injury can increase the risk of morbidity and mortality; therefore, eGFR should be the preferred screening method for detecting renal injury in them 3,6,29.
Table 3 shows that patients who progressed to death often did not recover from AKI during a follow-up. AKI represents a significant complication in the ICU, particularly affecting older patients, with an overall incidence rate ranging from approximately 20-40% among admitted patients.30-32 According to Vijayan et al.30, recovery of kidney function tends to be less frequent in older patients, especially those with multiple comorbid conditions at baseline. In a study conducted by Hoste et al., it was observed that AKI occurs in approximately one to two-thirds of patients hospitalized in the ICU.32 Among these patients, approximately 10 to 15% require support with renal replacement therapy (RRT) 32. Despite therapeutic and diagnostic advancements, the mortality rate for this patient group has remained constant in recent years, at approximately 50% 3,5,6,30-32.
Additionally, according to Machado Levi et al.33, even small changes in serum creatinine levels can be associated with increased mortality. Consistent with this fact, the present study demonstrated that patients who progressed to death had an average maximum creatinine value approximately three times higher than the initial value, whereas these values were much lower for patients who did not experience death as an outcome (Table 3).
Certain comorbidities or conditions can impair or intensify renal function impairment, such as diabetes mellitus and systemic hypertension, thereby elevating the likelihood of developing kidney disease 6,7. In the present study, older patients with AKI presented similar characteristics and clinical conditions as patients in other studies in the literature, such as advanced age and various comorbidities, including stroke, diabetes mellitus, hypertension, cardiovascular disease, and diverticular disease. In the multivariate analysis, significant variables associated with mortality included age, comorbidities, and BIS-creatinine results (Table 4).
According to Teles et al. 34, older individuals are more prone to developing AKI and have higher mortality rates than the general population. Evidence suggests that dialysis in older adult patients does not yield positive survival outcomes, despite being a therapeutic method for life maintenance, especially in patients with existing comorbidities.3,6,34,35 According to the mentioned study, for each additional year of life, the risk of death increases by 20%. In other words, the older the patient is, the higher the risk of mortality.3,6,8,34,35
As presented in Table 4, for each additional comorbidity a patient experienced, the likelihood for mortality increased (1.08 times higher), with hypertension contributing significantly to the risk of death, as shown in Table 2.
The evaluation of eGFR methods, CKD-EPI and BIS-creatinine, demonstrated differing performance, with BIS-creatinine exhibiting a stronger correlation with mortality in older adult patients admitted to the ICU who subsequently developed AKI 7, 36. This finding aligns with that observed by Beridze et al.36 in a cohort of 3,000 older adult patients in Sweden, with a mean age of 78 years.
This study has certain limitations, as it is a retrospective observational study conducted at a single centre with a small sample size and only four survivors. Most of the patients were not admitted to the ICU in a timely manner, as indicated by the high percentage of patients requiring mechanical ventilation and vasopressor administration. This suggests that many patients may have already had AKI at the time of admission and were consequently excluded from this study. Despite these limitations, this study represented a real-world setting with ICU patients in a low-resource region of Brazil, where a small number were admitted without renal injury.
The results observed in the multivariate analysis coincided with findings from the existing literature and raise awareness about the inadequacy of serum creatinine as a renal function marker in older individuals, thereby suggesting the incorporation of BIS-creatinine as a useful tool for monitoring older adult patients admitted to ICUs.