Patient characteristics
Totally, 2716 patients were eligible in this study. The mean (median, range) age of the patients was 71.6 (75, 63–83) years and most of the study population consisted of males (60.9%). 487 (17.9%) patients complicated with acute respiratory failure. 447 (16.5%) patients required mechanical ventilation, and 597 (22.0%) patients needed admission to ICU.
AKI characteristics
766 (28%) patients developed AKI. The characteristics of patients with AKI were shown in Table 1. Compared with non-AKI group, male gender (67.2% versus 58.4%; P < 0.001) and older age (78.2 years versus 69.0 years; P < 0.001) had significant difference between the two groups. Hypertension, diabetes mellitus, coronary artery disease, cardiac insufficiency, atrial fibrillation, chronic kidney disease, and cerebrovascular diseases were more common in the AKI group. However, no statistically significant comorbidities were in COPD, pulmonary hypertension, and tumour between the two groups. Patients in the AKI group were more commonly complicated with acute respiratory failure (39.2% versus 9.6%; P < 0.001), and had a higher CURB-65 score than the no-AKI group. In addition, patients with AKI had higher levels of uric acid, Cystatin C, and white blood cell count, while they had lower levels of albumin, hemoglobin, and platelet count.
Table 1
Patients baseline in patients with and without acute kidney injury
Variables
|
All
(n = 2716)
|
Non-AKI
(n = 1950)
|
AKI
(n = 766)
|
P Value
|
Demographics
|
|
|
|
|
Gender (male), n (%)
|
1653 (60.9)
|
1138 (58.4)
|
515 (67.2)
|
<0.001
|
Age (years)
|
71.6 ± 15.8
|
69.0 ± 16.2
|
78.2 ± 12.3
|
<0.001
|
Comorbid conditions, n (%)
|
|
|
|
|
Hypertension
|
1380 (50.8)
|
903 (46.3)
|
477 (62.3)
|
<0.001
|
Diabetes mellitus
|
553 (20.4)
|
346 (17.7)
|
207 (27.0)
|
<0.001
|
Coronary artery disease
|
779 (28.7)
|
481 (24.7)
|
298 (38.9)
|
<0.001
|
Cardiac insufficiency
|
618 (22.8)
|
335 (17.2)
|
283 (36.9)
|
<0.001
|
Atrial fibrillation
|
317 (11.7)
|
182 (9.3)
|
135 (17.6)
|
<0.001
|
COPD
|
336 (12.4)
|
234 (12.0)
|
102 (13.3)
|
0.349
|
Chronic kidney disease
|
184 (6.8)
|
71 (3.6)
|
113 (14.8)
|
<0.001
|
Pulmonary hypertension
|
87 (3.2)
|
57 (2.9)
|
30 (3.9)
|
0.186
|
Tumour
|
238 (8.8)
|
159 (8.2)
|
79 (10.3)
|
0.073
|
Cerebrovascular diseases
|
883 (32.5)
|
535 (27.4)
|
348 (45.4)
|
<0.001
|
Complication
|
|
|
|
|
Acute respiratory failure
|
487 (17.9)
|
187 (9.6)
|
300 (39.2)
|
<0.001
|
Severity scoring
|
|
|
|
|
CURB-65 scores
|
|
|
|
<0.001
|
0
|
587 (21.6)
|
561 (28.8)
|
26 (3.4)
|
|
1
|
1211 (44.6)
|
916 (47.0)
|
294 (38.6)
|
|
2
|
741 (27.3)
|
416 (21.3)
|
323 (42.4)
|
|
≥ 3
|
177 (6.5)
|
56 (2.9)
|
121 (15.8)
|
|
Laboratory tests
|
|
|
|
|
Albumin (g/L)
|
33.2 ± 5.3
|
34.2 ± 4.8
|
30.7 ± 5.4
|
<0.001
|
Uric acid (umol/L)
|
257 (186–360)
|
239 (178–314)
|
345 (228–477)
|
<0.001
|
Cystatin C (mg/L)
|
1.3 ± 0.7
|
1.1 ± 0.4
|
1.9 ± 0.9
|
<0.001
|
Hemoglobin (g/L)
|
118.4 ± 21.6
|
121.3 ± 19.5
|
111.0 ± 24.8
|
<0.001
|
Platelet count (109/L)
|
198 (148–255)
|
188 (147–228)
|
155 (121–203)
|
<0.001
|
White blood cell count (109/L)
|
7.4 (5.5–10.1)
|
6.9 (5.3–9.2)
|
8.8 (6.2–12.7)
|
<0.001
|
Abbreviation: AKI = acute kidney injury; COPD = chronic obstructive pulmonary disease; CURB-65 = confusion, urea nitrogen, respiratory rate, blood pressure, and age≥65 years.
Independent factors for AKI
Multivariate logistic regression analysis revealed that Cystatin C (OR 4.27, 95% CI 3.36–5.44; P < 0.001), acute respiratory failure (OR 3.96, 95% CI 2.29–3.83; P < 0.001), albumin (OR 0.91, 95% CI 0.89–0.94; P < 0.001), uric acid (OR 1.002, 95% CI 1.001–1.003; P < 0.001), platelet count (OR 0.997, 95% CI 0.996–0.998; P = 0.001), white blood cell count (OR 1.08, 95% CI 1.05–1.10; P < 0.001), and CURB-65 score were independent factors for AKI in patients with CAP. (Table 2)
Table 2
Independent factors of AKI in patients with CAP
Variable
|
OR
|
95% CI
|
P Value
|
Cystatin C (mg/L)
|
4.27
|
3.36–5.44
|
< 0.001
|
Albumin (g/L)
|
0.91
|
0.89–0.94
|
< 0.001
|
Uric acid (umol/L)
|
1.002
|
1.001–1.003
|
< 0.001
|
Platelet count (109/L)
|
0.997
|
0.996–0.998
|
0.001
|
White blood cell count (109/L)
|
1.08
|
1.05–1.10
|
< 0.001
|
CURB-65 scores
|
|
|
|
0
|
Reference
|
|
|
1
|
3.03
|
1.82–5.05
|
< 0.001
|
2
|
4.10
|
2.38–7.08
|
< 0.001
|
≥ 3
|
7.14
|
3.74–13.62
|
< 0.001
|
Acute respiratory failure
|
2.96
|
2.29–3.83
|
< 0.001
|
Abbreviation: AKI = acute kidney injury; CAP = community acquired pneumonia; OR = odds ratio; CI = confidence interval; CURB-65 = confusion, urea nitrogen, respiratory rate, blood pressure, and age≥65 years.
Prediction efficiency of Cystatin C for AKI in patients with CAP
We performed ROC analysis for all independent factors for AKI to determine their prediction performance, respectively. Figure 2 showed the comparisons of AUCs for all independent factors of AKI in patients with CAP. The maximum AUC was reported for serum Cystatin C within 24 hours after admission. Table 3 presented the accuracy of serum Cystatin C for detecting AKI in patients with CAP. Cystatin C had an AUC of 0.81 (95% CI: 0.79–0.83, P < 0.001) for predicting AKI, with an optimal cut-off value of 1.37 mg/L, computing 68% sensitivity, 80% specificity, 57% positive predictive value and 86% negative predictive value.
Table 3
The accuracy of serum Cystatin C for detecting AKI in patients with CAP
AUC 95% CI
|
Cut-off point
|
Sensitivity
|
Specificity
|
Accuracy
|
PPV
|
NPV
|
0.81 95% (0.79-0.83)
|
1.37 mg/L
|
68%
|
80%
|
77%
|
57%
|
86%
|
Abbreviation: AKI = acute kidney injury; CAP = community acquired pneumonia; AUC = area under the receiver operating characteristic curve; CI = confidence interval; PPV = positive predictive value; NPV = negative predictive value.
Subgroup analysis
In the stratification analyses, serum Cystatin C within 24 hours after admission was still an independent predictor in all the various subgroups. Figure 3 showed the ORs, AUCs, cut-off values, sensitivity, and specificity of Cystatin C in all the different subgroups. Moreover, Cystatin C still had a good performance for predicting AKI in all the subgroups (AUCs: 0.69-0.84).