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Background
Role of point of care ultrasound in management of acute kidney injury (AKI) is still unclear. The discrimination between AKI and chronic kidney disease (CKD) in ED, especially in the absence of clear CKD history, represents a dilemma.
Aim
To test the accuracy of POCUS and intact parathormone hormone (iPTH) as differentiators between AKI and CKD, and to investigate the role of POCUS in management of AKI in the ED.
Methods
This prospective study involved 95 adult patients presented to ED of Alexandria Main University Hospital, with signs and/or symptoms suggesting renal impairment in the absence of prior renal functions tests from April 2022 to December 2022. POCUS was done and iPTH was measured for all enrolled patients. Validity of renal length and iPTH to discriminate AKI and CKD were tested. POCUS were used to identify different underlying causes of AKI.
Results
Renal length ≤ 9.62 cm could diagnose CKD (Area under curve-AUC 0.926) with 82.46% sensitivity, 92.11% specificity, 94.0% PPV and 77.8% NPV. Intact PTH level > 161 pg/ml could diagnose CKD (AUC 0.844) with 73.17% sensitivity, 92.11% specificity, 90.9% PPV and 76.1% NPV.
Conclusion
Both renal length and iPTH are accurate tests to differentiate AKI from CKD in ED. POCUS help identifying the different causes of AKI and reaching appropriate management decisions.
1. Introduction
Point of care ultrasound (POCUS) performed by the emergency physician (EP) is considered one of the main pillars of patient management in emergency medicine (EM) [Citation1–4]. Studies have tested the competency of EP to perform POCUS, with recommendations from various societies of EM supporting EP training on POCUS and approving many protocols for different emergency conditions in the emergency department (ED) [Citation5–7]. One of unclear issues is the role of POCUS in acute kidney injury (AKI), which is considered one of the most common diagnoses in the ED and carries high mortality and morbidity [Citation8–10]. The incidence of AKI in developing countries reaches up to 60% among critically ill patients [Citation11,Citation12]. At the same time, the development or progression of chronic kidney disease (CKD) after having one or more attacks of AKI has great socioeconomic and public health effects with increased healthcare costs in both high- and low-income countries [Citation13]. AKI is differentiated from CKD by its rapid and reversible deterioration of renal functions [Citation14]. Kidney Disease Global Improvement Outcome (KDIGO) guidelines define AKI as an abrupt decrease in kidney function diagnosed by an increase in serum creatinine (SCr) by ≥0.3 mg/dl within 48 h, an increase in SCr to ≥ 1.5 times baseline within 7 days or a urine volume <0.5 ml/kg/h for 6 h [Citation15].
Intact parathormone hormone (iPTH) as a laboratory biomarker could solve the problem of differentiating AKI and CKD, as it increases in CKD as a result of secondary hyperparathyroidism and is recommended to be used in the monitoring of CKD patients [Citation16,Citation17]. In addition, iPTH was also found to be increased in AKI which enhances its potential role as a discriminator [Citation18,Citation19]. POCUS can also play an important role in differentiating AKI from CKD especially if baseline renal function tests are not available. Additionally, POCUS may help in the rapid identification and management of different etiologies of AKI, especially that rapid identification and management of reversible causes of AKI are highly recommended [Citation15].
2. Patients and methods
95 adult patients presented to the ED of Alexandria Main University Hospital in Egypt were prospectively enrolled to the study from April 2022 to December 2022. Informed consent was obtained from all patients or next of kin. Eligible patients were adults with signs and/or symptoms suggesting renal impairment; oliguria (<0.5 ml/Kg/hr), anuria, abnormal renal function test, electrolyte disturbance, metabolic abnormalities and/or volume overload, with absent baseline renal functions. We excluded pregnant patients, patients with history of parathyroid disease or surgery or patients on calcium or phosphate binder treatment. All eligible patients underwent both POCUS and departmental ultrasound (by radiologist). POCUS was done by specialist of EM, experienced in POCUS and he was not blinded to the results of patients’ examinations and investigations. Both radiologist and POCUS physician were blinded to results of each other. The results of ultrasound scans were documented and reviewed with the final diagnosis by an observer who was blind to both ultrasound scans results. Data collection included demographic characteristics, clinical and laboratory data. A serum sample for iPTH was taken for all enrolled patients and was analyzed using Elecsys PTH stat 2010 and Cobas-e 411 (Roche) with a normal reference range of 1565 pg/ml.
2.1. POCUS examination
POCUS was carried out using different ultrasound machines according to availability: Mindray DP-5 and DP-20 and Toshiba Xario 200 TUS-X200. Curvilinear or phased array probes with low frequencies (2–5 MHz) were used for renal and abdominal examinations. For cardiac examination, a phased array probe was used, while a high-frequency linear probe (5–12 MHz) was used for lung ultrasound. Patients were routinely examined in the supine position, although the lateral position was sometimes used for better imaging. Renal ultrasound (RUS) was performed in two planes, longitudinal and transverse, and includes measuring renal length and assessment of cortical echogenicity and corticomedullary differentiation (CMD). Renal length was measured from pole to pole, assuming the normal renal length from 10 to 12 cm, and shorter length was assumed as CKD [Citation20]. Cortical echogenicity assessed qualitatively in relation to liver and spleen [Citation21]. Qualitative assessment for left and right ventricular functions was done by the eyeballing method, also pericardial effusion or tamponade were also assessed [Citation22,Citation23]. The inferior vena cava (IVC) was assessed with B and M mode in the subcostal window at 2 to 3 cm distal to its entrance to the right atrium to estimate the intravascular volume, both IVC diameter and collapsibility index were calculated [Citation24,Citation25].
POCUS was used to differentiate between AKI and CKD and to identify AKI causes (pre-renal, renal and post-renal). IVC assessment used to differentiate between fluid depletion and overload. POCUS findings and clinical parameters were used together to identify different types of shock, where shock was defined as SBP <90 mm Hg, or shock index ≥ 1 [Citation26]. Cardiorenal cause was considered if signs of acute decompensated heart failure present: decreased contractility, plethoric IVC, pulmonary B lines [Citation27]. Lung ultrasound was performed to identify signs of overload, congestion (B lines, pleural effusion) or pneumothorax (absence lung sliding and stratosphere sign) [Citation28]. The Possibility of renal causes was considered if clear history of using nephrotoxic drugs, evidence of renal infection and acute renal ischemia were present. Hydronephrosis and its possible causes were identified by US as obstructive causes of AKI [Citation29]. ()
Figure 1. Different POCUS findings: (a) short hyperechogenic right kidney reflecting CKD; (b) B and M modes of IVC assessment show plethoric IVC; (c) hyperechogenic kidney with moderate degree of hydronephrosis; (d) lung ultrasound shows lung congestion (B lines).
3. Statistical analysis
Data were entered into IBM SPSS software package version 20.0 for statistical analysis. Qualitative data were described using numbers and percent. Quantitative data were described using mean, standard deviation (SD), median and interquartile range (IQR). The Shapiro–Wilk test was used to verify the normality of distribution, and the significance of the obtained results was judged at the 5% level. Chi-square test was used to compare categorical variables. F-test (ANOVA) and Post-Hoc test (Tukey) for pairwise comparisons and Kruskal‒Wallis test and Post-Hoc (Dunn’s multiple comparisons test) for pairwise comparisons were used to compare between quantitative variables in more than two studied groups. Receiver operating characteristic curve (ROC) was used to investigate the diagnostic performance of iPTH and renal length as discriminators, with area under curve (AUC) greater than 50% indicating acceptable performance and an area about 100% indicating the best performance for the test. Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) were calculated. The minimum sample needed was 80 patients to achieve significance level of 5% and a power of 80% with an assumption of 80–95% sensitivity of the test [Citation30].
4. Results
95 adult patients were enrolled in the study, 50 were females (52.6%) and 45 were males (47.7%). The median of age was 69 (IQR 62–75) years and mean of Body Mass Index (BMI) 28.61 ± 6.52 Kg/m2. Patients were classified according to final diagnosis into three groups: AKI (group 1, n = 38), AKI on top of CKD (group 2, n = 41) and CKD (group 3, n = 16). No statistical significance differences regarding demographic characteristics between groups were present. However, the hemoglobin and serum calcium levels were significantly lower in groups 2 and 3, in comparison with group 1 (p 0.004 for both), while serum creatinine and phosphorus levels were lower in group 1 in comparison with group 2 and 3 (p 0.002 and < 0.001, respectively). These parameters were not significantly different between groups 2 and 3. Additionally, the level of iPTH was elevated among the three groups, but was lower for group 1 in comparison with other two groups: Median of iPTH in group 1 was 94.9 pg/ml (1QR 80.1–122), while in group 2 median was 230 pg/ml (IQR 152–345) and 204 pg/ml (IQR160.7–297) in group 3 (p1&2 <0.001). ()
Table 1. Baseline patients’ characteristic and laboratory results total sample and in each group.
Regarding RUS, the median of renal length was statistically significant different between groups, where shorter renal length was found in both group 2 (8.6 cm and IQR 7.9–9.5) and group 3 (9.13 cm, IQR 8.6–9.4), while the difference between these two groups was not significant (p3 = 0.647). On the other hand, the renal length for group 1 was 11.2 cm IQR 10.5–12.3, significantly different from other groups (p1&2 <0.001). Cortical echogenicity and CMD were not statistically significantly different between group 2 and 3, but the difference was significant with group 1. Most of patients in group 1 had normal cortical echogenic (>70%), while ≥ 25% had grade I echogenicity, one case grade II, and no cases were grade III. On the contrary, most of cases in CKD groups (2&3) had increased echogenicity, especially grades II and III (p < 0.001). CMD was preserved for most of patients in group 1 in comparison with group 2 and 3 (p < 0.001) but not significantly different between group 2 and 3. ()
Table 2. Renal ultrasound results among the studied groups.
By Comparing AUC for renal length and iPTH, both showed excellent performance, but the renal length found to have outstanding performance as differentiator between AKI and CKD with AUC = 0.934 (95% CI 0.885–0.983), AUC for iPTH was 0.844 (95% CI 0.764–0.925). A cut-off value ≤9.62 cm of renal length could diagnose CKD with 82.46% sensitivity, 92.11% specificity, 94.0% PPV and 77.8% NPV (). Additionally, cut-off value of iPTH >161 pg/ml could diagnose CKD and discriminate it from AKI with 73.17%, sensitivity, 92.11%, specificity, 90.9% PPV and 76.1% NPV. ()
Figure 2. ROC curve for renal length to differentiate CKD from AKI, AUC 0.934 (95% CI 0.885–0.983).
Figure 3. ROC curve for intact parathormone hormone to differentiate CKD from AKI, AUC was 0.844 (95% CI 0.764–0.925)..
No significant differences between AKI and AKI on top of CKD in results of POCUS regarding the underlying causes of AKI were found. 50% of AKI patients (group 1 and 2) were found to have decreased IVC diameter with increased CI (>50%), while about quarter of them have plethoric IVC. Decreased cardiac contractility was diagnosed in one third of cases, while only in 11% assumed to be the cause of AKI. B lines were present in about 40% of patients, while around 32% were having pleural effusion ().
Table 3. Point of care ultrasound findings in AKI patients.
The pre-renal causes of AKI were the most common causes identified (92.4%), mostly due to decreased oral intake (69.6%) or hypo-perfusion (24.1%). Sepsis was a pre-renal factor in 67.1% patients and was mixed with hypovolemia and decreased oral intake in 59.5% of cases. 13.9% patients were having obstructive causes, where stones were the most common cause. 5.1% of cases were considered having renal causes: nephrotoxic dugs, pyelonephritis or renal ischemia. ()
Table 4. Causes of AKI among AKI groups; AKI and AKI on top of CKD.
5. Discussion
In this study, we offered solutions for a common dilemma of distinguishing acute and chronic renal impairment in ED, especially when the baseline renal function tests are unavailable. The renal length was not surprisingly to be short in CKD, but its outstanding performance as discriminator makes it an excellent choice. Although both tests could be used but the higher performance of renal length (AUC 0.934 versus 0.844) makes it the first choice. Intact PTH might help in certain conditions; CKD is associated with normal or increased renal length. In the present study, a cut-off value of renal length ≤9.62 cm diagnoses CKD with high sensitivity. Ozman et al., [Citation20] set a cut-off 7.1 cm (AUC 0.865) with 100% sensitivity and specificity, which is shorter than our results. We agreed with this study in that short length kidney is diagnostic for CKD, but we does not agree with setting shorter cut-off value for diagnosing CKD. As for patients not previously diagnosed as CKD (not complained), the probability of having very short kidney may be uncommon and can limit the use of renal length as a discriminator.
In our study, we found that although the increased cortical echogenicity commonly was found in CKD (p < 0.001), but there is still grey zone in this aspect, makes the depending on the increased cortical echogenecity alone to diagnose CKD (except for higher grades of CKD) can’t accurately differentiate AKI and CKD, as grade I echogenicity and to lesser extent grade II were present in quarter of AKI cases; hence increase echogenicity can’t accurately alone exclude AKI. O’Neill et al. [Citation31] explained different situations of AKI, where kidney shows increased echogenicity as in ischemic acute tubular necrosis and glomerulonephritis. Also, Ozmen et al. [Citation20] concluded that echogenicity has poor performance as discriminator between AKI and CKD. In general, short kidney with increased echogenicity usually diagnose CKD, while normal length and echogenic or slight increased echogenic kidney indicating less severe renal condition coinciding with diagnosis of AKI condition.
Parathormone hormone was found to be increased in both AKI and CKD secondary to hyperparathyroidism accompanying renal impairment [Citation32,Citation33]. Unlike the traditionally measured whole PTH assay, intact PTH is not metabolized by kidney, so its results will not be affected by the renal impairments. As renal length, the iPTH level could differentiate AKI from CKD but could not discriminate between AKI on top of CKD or CKD. The cut-off level >161 pg/ml could diagnose CKD with good sensitivity (73.17%) and high specificity (92.11%). In the study by Ozmen S et al. [Citation18], similar results were concluded and found that a cutoff >170 pg/ml of iPTH could discriminate CKD (AUC 0.66, 88% sensitivity and 89% specificity).
Similar previous studies (as previously mentioned) focused on either POCUS role in discriminating AKI and CKD or identifying the obstructive causes of AKI [Citation34]. In this study, we extended the role of POCUS using POCUS assessment (IVC, heart, lung, abdomen and kidney assessment) with the clinical parameters to identify the causes of AKI and help reaching appropriate medical decision.
In the current study, POCUS protocol was used to identify variable underlying causes of AKI in addition to the presence of multiple factors simultaneously. Pre-renal causes were the most common (92%) factors identified, and wherein 72.2% of cases, we identified the presence of more than one factor together; hypovolemia and volume depletion were common factors present together and in more than 50% of cases accompanied by sepsis. These results agreed with Goswami et al., [Citation35] as in that study they found that most common cause for community acquired AKI (CA AKI) in developing countries is the pre-renal causes, where volume depletion and hypo-perfusion are most underlying factors. Although the previous study stated lower percentage than us, but this may be because they had used different categorizations for the etiological factors, also they did not use POCUS in the assessment and did not state clearly the existence of multifactor for AKI simultaneously. As example of the presence of pre-renal, renal and post-renal factors simultaneously in same case, where POCUS diagnosed bilateral pyelonephritis (renal cause), hydronephrosis (obstructive cause) and volume depletion (pre-renal), through IVC assessment.
Diagnosing hydronephrosis by ultrasound and the accuracy of its identification by non-radiologist has been tested [Citation36,Citation37]. In this study, hydronephrosis was identified in 13.9%, these results agreed with the international prevalence of post-renal AKI (10%) [Citation38]. Nepal S [Citation37] stated that the post-renal AKI is around 8.5%, while Goswami S [Citation35] said that it reached 9% in the developing countries.
Diagnosing renal structural damage as a cause of AKI (renal causes) usually needs advanced investigations as renal biobsy for histopathological examination to confirm the diagnosis of acute tubular necrosis or in some situations more advanced laboratory investigation as antiglomular basment membrane or immunological assays to diagnose renal causes of AKI which not routinely performed in ED, but with ultrasound we could diagnose some causes as ischemic or infective renal causes, in all diagnosed renal cases (four cases) clear history of nephrotoxic drugs or evidence of intrarenal infection were present.
Ultrasound assessment of IVC (diameter and collapsibility) was used as surrogate for intravascular volume assessment to differentiate if patient was hypovolemic, euvolemic or overloaded [Citation39,Citation40]. The identification of overload helps in avoiding the fatal consequences of giving fluids (if was overloaded) for suspected AKI patient with oliguria or anuria in ED, also IVC could help as noninvasive monitoring of the fluid status to avoid over-hydration in refractory AKI. IVC assessment in addition to data obtained from the focused cardiac ultrasound assessment and lung and abdominal ultrasound examinations provided great clues for identifying the underlying factors and optimizes the medical decision.
We offered a simple POCUS approach for emergency physicians for management of AKI in ED, which is not a replacement of the comprehensive ultrasound scan or other radiological and laboratory investigations.
The present study is limited by a relatively small convenience sample size of patients where POCUS performed by single operator; therefore, inter-rater reliability of the protocol could not be tested.
6. Conclusion
According to this study, incorporating POCUS in the management of AKI in ED could add great value for patients in rapidly identifying most of AKI etiologies and reaching appropriate medical decision that could improve patients’ outcomes. Both POCUS and iPTH could accurately differentiate AKI and CKD.
Availability of data and material
Data generated and/or analyzed in this study are available from the corresponding author upon reasonable request.
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