Results
During the study period, there were 113, 723 adult admissions, of which
15,114 (13.3%) were not eligible for this analysis and a further 28,372
(25.9% of the total) were excluded because they had no data on SCr
during hospitalisation (Figure 1). After a random selection of one
admission per patient, the final study data set included 48,835
patients. Ambulatory preadmission SCr was ascertained in 20.7% of
admissions; for the remaining the baseline SCr was imputed.
AKI incidence
AKI was identified in 7,427 patients giving an overall incidence of
15.2%. In 2,847 patients, criteria of AKI were fulfilled within the
first 24 hours after admission, accounting for 5.8% of all hospital
admissions and 38.3% of all AKI episodes. In 4,580 patients, AKI was
not present within the first day, but developed later during their stay,
with the earliest SCr keeping with the definition, on average, about
five days after admission, irrespective the HA-AKI stage (p=0.05)
(Figure 2).
Patients’ characteristics
In comparison to No AKI patients, those with AKI were older, more likely
to be admitted via the emergency room, had more comorbidities and more
often required critical care (Table 1).
Patients with CA- and HA-AKI had a very similar distribution of
comorbidities, except for CKD, which we found more prevalent in those
with AKI at admission. The type of AKI also related to the principal
diagnosis, with genitourinary system and infectious diseases being most
frequent in CA-AKI and circulatory system disease in HA-AKI patients.
Serum creatinine at admission was, on average, twice as high as the
baseline value in patients with AKI at admission. These patients were
also more likely to endure more severe AKI (23.2% stage 2 and 30.9%
stage 3 in CA-AKI vs 21.4% and 14.5%, in HA-AKI, respectively).
Outcomes
Greater severity of AKI when apparent at admission translated into a
higher number of patients requiring in-patient RRT and slightly higher
in-hospital mortality (8.3% and 23.3%, respectively, vs 3.9% and
20.6% in HA-AKI) (Table 1). Patients with CA-AKI had a median length of
hospital stay of 8.1 days (P25, P75: 4.8, 14.4) which was shorter, on
average, almost seven days than a length of stay in HA-AKI group. In the
latter, significant differences in the total length of stay between
severity of AKI (p<0.001) arose from the number of days after,
not the number of days before AKI detection (Figure 2). Among patients
who survived hospitalisation, 6.4 % of CA-AKI and 5.3% of HA-AKI died
up to 6th-month after discharge.
Generally, the outcomes of our interest worsened according to staging,
whether AKI was present at admission or acquired during hospital stay
(Figure 3). One should notice that an initially low in-hospital
mortality in HA-AKI stage 2 and 3 started steadily increasing and in the
post-discharge period exceeded the mortality rate observed in the group
of most severe CA-AKI.
After multivariable adjustment in the Cox model, the gradual
relationship between the severity of AKI and the risk of the adverse
outcome remained significant (Table 2). Patients with HA-AKI stage 1
were least likely to die during hospitalisation among all AKI patients,
but still, over twice more likely than No AKI patients (hazard ratio
(HR) 2.28, 95% confidence interval (CI): 2.03–2.56). Patients in stage
3 of both CA- and HA-AKI were in a much higher risk of death, either
during hospital (HR 5.65, 95% CI: 4.81–6.63 and HR 6.70, 95% CI:
5.84–7.68) or within six months of discharge (HR 2.50, 95% CI:
1.79–3.49 and HR 2.18, 95% CI: 1.53-3.11), than patients in other
stages.
Hazard function
The survival regression with the Weibull distribution of time to AKI
occurrence coincided better with the function determined by Kaplan-Meier
curve and showed a better fit to our data in comparison to the model
with an exponential distribution of time (AIC 47437 vs 47588) (Figure 4
A). The probability of AKI occurring in subsequent days decreased from
1.68 in the first day to 1.27 in the 30th day of hospitalisation (Figure
4 B).