Elevation of serum phosphorus, an early biomarker of acute kidney injury after cardiac sugery?

Introduction

Acute kidney injury (AKI) is common after cardiac surgery and is a strong predictor of morbidity and mortality [1]. Hyperphosphatemia following AKI, by renal excretion defect, has never been studied in this context and could be a simple marker of AKI.

Objectives

The aim of this study was to assess the predictability of serum phosphorus (Ph) for AKI monitoring after cardiac surgery.

Methods

In this retrospective diagnostic validation study of 547 patients admitted in our institute between January 2012 and December 2012, we excluded patients with end stage renal disease (clearance < 15mL / min / 1.73m2) or dialyzed, solitary kidney or nephrectomy, lack of data. Serum creatinine (Cr) and Ph were measured preoperatively and postoperatively specifically (H0, H12, H24, H48, H72). The percentage of maximum elevation of Ph (%EPh = [(maximum -minimum) / minimum] * 100) was calculated. AKI was defined as an increase Cr more than 26.5 mmol / L in 48 hours according to KDIGO criteria [2].The diagnostic performance of postoperative Ph thresholds were analysed by elaborating area under the receiver operating characteristic curves (AUC-ROC) with sensitivity (Se), specificity (Sp), positive predictive value (PPV), negative predictive value (NPV).

Results

From the 386 patients included, the mean Euroscore II was 4.2 ± 6.3%, SAPS II score, 26.4 ± 10.8. Among them, 21.2% developed AKI (grade 1: 13.2%, grade 2: 4.1%, grade 3: 3.1%) and 2.6% required renal replacement therapy (RRT). Patients with AKI had Euroscore II, duration of cardiopulmonary bypass, transfusion needing and mortality higher than those without AKI (p < 0.001). The %EPh and the Ph at 48 hours (Ph_48H) were significantly higher in AKI patients than in no AKI patients: 81 ± 79% and 1.47 ± 0.46 mmol/l vs 25 ± 23% and 0.99 ± 0.2 mmol/L, respectively (p < 0.001). A value of Ph_48H > 1.19 mmol/L ( Se 72% (60-82), Sp 84% (71-92), PPV 84%, NPV 72%) and a %EPh > 49 % (Se 73% (61-81), Sp 83% (76-88), PPV 66%, NPV 86%) were predictive of AKI. In AKI patients, the %EPh and Ph_48H significantly increased with the severity of AKI (Table 1). In these patients, a Ph_48H < 1.53 mmol/L and a %EPh < 77% predicted the non use of RRT (Se 100% (62-100), Sp 85% (77-91), PPV 35% NPV 100%), respectively (Table 2).

Table 1

Table 2

Conclusions

After cardiac surgery, serum phosphorus seems to be a simple, reliable and inexpensive biomarker at bedside for AKI monitoring. A value less than 1.53 mmol/L at 48h may predict the no-initiation of RRT in case of AKI and may guide the clinician to a non-invasive-AKI therapeutic. Obviously, these results should be interpreted with caution regarding the retrospective nature of the study.