The procalcitonin-to-cortisol ratio is a potential prognostic predictor in sepsi

Hui Liu, Jie Hu, Jian-guo Xiao, Hong-jun Kang, Fei-hu Zhou

Department of Critical Care Medicine, the First Medical Center, Chinese PLA General Hospital, Beijing 100853, China

KEYWORDS: Sepsis; Cortisol; Procalcitonin; Procalcitonin-to-cortisol ratio; Adrenal insufficiency

INTRODUCTION

Corticosteroids are elevated in response to the stress triggered by sepsis.[1]However, assessing the severity of“relatively insufficient” is an unsolved problem. In 2008,the Society of Critical Care Medicine (SCCM) clarified the concept of critical illness-related corticosteroid insufficiency (CIRCI). In particular, CIRCI is different from primary endocrine diseases. CIRCI was not an absolute corticosteroid insufficiency but a relative “starving situation” for more cortisol even when the cortisol level had elevated. However, it is challenging to measure CIRCI because cortisol replacement therapy is unsuccessful and full of controversies. Among plenty of sepsis biomarkers,procalcitonin (PCT) is reported to represent the severity of infection with a high specificity.[2]Accordingly, the procalcitonin-to-cortisol ratio (P/C ratio) was expected to reflect the response capacity to stress of infection. In our hypothesis, high P/C ratio might indicate an insufficient response capability, which will likely result in a worse prognosis.

Relative adrenal insufficiency occurs in more than 50% of severe sepsis patients and causes excessive inflammation, immune exhaustion, and a worse prognosis.[3]However, the definition of CIRCI remains controversial. Furthermore, it is unclear how much cortisol is required to inhibit excessive inflammation during sepsis. Therefore, the present retrospective clinical cohort study was conducted to explore a new prognostic predictor for sepsis and investigate CIRCI.[4]

Early stress and inflammatory responses were examined in septic post-surgery patients with abdominal sources in the present study. Cortisol, PCT, and other parameters were detected two days after sepsis onset.PCT was selected to assess abdominal infection severity because a lack of adrenal stress hormones impairs the capacity of the body to deal with stress adequately,leading to higher mortality. P/C ratio was used as a combined biomarker to represent the stress response capacity for infection. In addition, the correlation between P/C ratio and prognosis was also analyzed.

METHODS

Study design

This retrospective observational study was conducted in our 22-bed general ICU over 15 months.This study protocol was approved by the Research Ethics Committee of Chinese PLA General Hospital (S2017-054-01).

Study population

One hundred thirty-two septic post-surgery patients hospitalized in the Department of Critical Care Medicine from September 2019 to December 2020, including 77 males and 55 females, were enrolled in this study.According to the American College of Chest Physicians/Society of Critical Care Medicine consensus conference,sepsis is defined as an acute increase in SOFA score of two or more from baseline in a patient with suspected infection.[5]Sepsis of the abdominal source was due to colon obstruction, colon fistulation, intestinal puncture,and anastomotic fistulation, among others. These septic patients were transferred to the ward after surgery.The inclusion criteria were as follows: (1) diagnosed according to Sepsis 3.0; (2) aged≥18 years and ≤90 years old; (3) without any endocrine diseases; and (4)sepsis caused by abdominal infection. The exclusion criteria were as follows: (1) received corticosteroid treatment that might impact cortisol measurement in the study; (2) survived less than 3 d after admission; (3) had high paraplegia; (4) had severe cerebral injury; (5) had liver failure; (6) pregnant patients; or (7) patients with chronic renal failure requiring hemodialysis or peritoneal dialysis. The enrollment workflow is shown in Figure 1.

Figure 1. The enrollment flowchart of this study.

Determination of the total serum cortisol

On the second day after sepsis onset, at 8 a.m.,blood samples were collected and analyzed for serum cortisol. Before the test, the plasma samples were stored between 2 °C and 4 °C. Total serum cortisol was measured by chemiluminescent immunoassay, performed on an ADVIA Centaur immunoassay system (Siemens,Ireland). The standard reference value of cortisol (8 a.m.)was 198.7-797.5 nmol/L.

Clinical parameters collection

On the second day after sepsis onset, age, body temperature, renal function, oxygen index, and other parameters were collected for calculating APACHE II and SOFA scores.

PCT, interleukin-6 (IL-6), body mass index(BMI), and C-response protein (CRP) were retrieved.Serum was separated within two hours after clinical blood collection and examined in the Laboratory Department. PCT was analyzed using a Cobas601 automatic electrochemiluminescence immunoanalyzer(Roche, Switzerland). CRP was detected through immunoturbidimetry with Image protein analyzer(Beckman Coulter, USA). IL-6 was examined with Immulite 1000 automatic chemiluminescence immunoanalyzer (Siemens, Ireland). The length of ICU stay, MV days, shock days (with vasopressor usage), and 28-day mortality were recorded.

Outcomes

The primary outcome was the 28-day all-cause mortality after sepsis diagnosis.

Statistical analysis

All the analyses were performed using R version 4.1.1. Continuous variables are expressed as the median(interquartile), and categorical variables were used in the stratified analysis. Independent samples, nonparametric rank-sum test and multivariable logistical regression were also used. The ratio of the patients was assessed by the Chi-square test. The cut-off points for sensitivity and specificity of P/C ratio in predicting mortality were estimated by receiver operator characteristic (ROC)curve analysis. Continuous variables between two groups were compared using the Mann-WhitneyUtest. The risk factors of sepsis mortality were evaluated using logistic regression analyses through independent variables. The relationship between P/C ratio and 28-day mortality was demonstrated by the restricted cubic spline (RCS) graph.AP-value ＜ 0.05 was considered statistically significant.

RESULTS

Baseline characteristics and outcome data of patients

One hundred thirty-two septic patients with abdominal infections were enrolled. Between the survivors and the non-survivors, there was no significant difference in age, gender, body mass index (BMI), CRP,length of ICU stay, or MV days. A general comparison of the baseline data, including cortisol, PCT, P/C ratio,IL-6, SOFA, shock days, and APACHE II, revealed a significant difference (Table 1). However, no significant difference was observed between the two groups in the source of abdominal infections (Table 1). In the survival group, shock days were shorter (7 d vs. 22 d,P＜0.001)due to the earlier reverse of hypotension in this group.

Logistic regression analysis of P/C ratio and its association with 28-day mortality outcome

Binomial logistical regression was applied to investigate the best predictor for 28-day mortality among SOFA, APACHE II, and P/C ratio. The results showed that P/C ratio and SOFA were significantly related to 28-day mortality (P=0.033 andP=0.004) (Table 2).

Stratified analysis and interaction modifiers based on potential risks

Stratified and interaction effect analyses were conducted to discover the influence in subgroups and the interaction between P/C ratio and other variables.Continuous variables were divided into two or three subgroups of different grades. As shown in Table 3, stratified analysis revealed that some subgroups significantly affected mortality. Meanwhile, no significant interaction was observed between P/C ratio and other variables (age, IL-6, CRP, SOFA, APACHE II)(P＞0.05, Table 3).

Demonstration of prognostic capacity by ROC curve analysis

Cortisol, PCT, and P/C ratio were selected for further ROC curve analysis. The results showed that P/C ratio had the highest area under the curve (AUC) value (Figure 2). The AUC of P/C ratio was 0.919 (95%CI: 0.870-0.970,P＜0.001). In addition, the AUC of cortisol was 0.506, and that of PCT was 0.876. The cut-off value of P/C ratio was 11.94, which indicated a survival outcome(Table 4).

Table 1. Baseline patients characteristics and outcome data

Table 2. Logistical regression analysis of factors for 28-day mortality

Table 3. Stratified analysis of characteristics and interaction modifiers

Table 4. Receiver operator characteristic curve analysis of cortisol, PCT and 1/CPR

Correlation between P/C ratio and survival possibility visualized through RCS analysis

RCS analysis was conducted to visualize the correlation between P/C ratio and survival possibility through smooth curve fitting. In this multivariate analysis, the dependent variable was the survival possibility, and the independent variable was 1/(P/C ratio). SOFA, APACHE II, CRP, and IL-6 were adjusted during RCS analysis. The survival possibility elevated when 1/(P/C ratio) increased. However, the positive correlation became weak, and the curve almost stopped rising when 1/(P/C ratio) was more than 0.2 (Figure 3).

Figure 2. ROC curve analysis of cortisol, PCT and P/C ratio. PCT:procalcitonin; P/C ratio: procalcitonin-to-cortisol ratio.

Figure 3. Smooth curve fitting of correlation between 1/(P/C ratio)and the surviving possibility by restrict cubic spline (RCS) analysis(adjusted by SOFA, APACHE II, CRP, and IL-6). SOFA: Sequential Organ Failure Assessment; APACHE II: Acute Physiology and Chronic Health Evaluation II; CRP: C-reaction protein; IL-6: interleukin-6.

DISCUSSION

Our research provides a potential index to assess CIRCI and predict the prognosis of sepsis. P/C ratio was verified to be a prognostic predictor in sepsis with an abdominal source in the present study. P/C ratio had a higher AUC than cortisol or PCT (P＜0.05) in the ROC analysis of 28-day hospital mortality. Moreover, RCS analysis demonstrated a good correlation between the 28-day survival possibility and P/C ratio. More than 85%of patients survived when 1/(P/C ratio) was ＞ 0.2.

It is clear that CIRCI is widespread in critical patients and is associated with worse prognosis.[3]However,

we are unable to identify CIRCI patients accurately.There are no practical biomarkers to date. CIRCI patients probably suffer from fatigue, nausea, vomiting,lethargy, or hypotension. However, these symptoms are not specific to diagnose CIRCI. The current laboratory diagnostic methods include a direct cortisol measurement test and an adrenocorticotropic hormone(ACTH) stimulation test. In 2017, the CIRCI guideline was updated by the America/Europe SCCM.[6,7]The high-dose ACTH (HD-ACTH) test became the standard method in this guideline. The HD-ACTH test was more likely to be the gold standard test for diagnosing primary adrenal insufficiency than critical illness.[7]On the other hand, an ACTH stimulation test is invalid for assessing adrenocortical integrity and function in critically ill patients due to confounding by many interference factors.The factors include dysfunction of the hypothalamicpituitary-adrenal (HPA) axis, glucocorticoid (GC)resistance, low albumin level, blood capillary leakage,increased cortisol distribution volume, or impaired cortisol clearance.[8]Cortisol measurements are affected by all of these factors. Therefore, the CIRCI diagnosis test has not been taken as a routine examination before administering cortisol in critical illnesses.[9]Furthermore,the HD-ACTH test was not recommended to screen CIRCI in clinics in the 2021 Surviving Sepsis Campaign Guideline.[10]

There is no effective method to target CIRCI patients in clinical trials, and there are no coincident conclusions about corticosteroid therapy. Meanwhile, conflicting results have been reported in different studies. For example, two recent clinical trials on steroids therapy in sepsis yielded mixed results. A trial of hydrocortisone and fludrocortisone demonstrated a mortality benefit within the first 24 h of sepsis (43.0% vs. 49.1%, relative risk [RR]=0.88, 95%CI:0.78-0.99).[11]In contrast, the second trial assessed hydrocortisone by continuous infusion, depicting no benefit on mortality (27.9% vs. 28.8%,OR=0.95,95%CI:0.82-1.10).[12]Both trials recruited patients using vasopressors, but doses of norepinephrine were higher in the positive study, suggesting that the early use of steroids in more severe patients could reduce mortality.Thus, the SCCM guidelines recommend hydrocortisone intravenous injection for septic shock unresponsive to fluid resuscitation.[13,14]

Our study showed that P/C ratio had a better predicting capability and shed more light on the understanding of CIRCI. In addition, the AUC of cortisol was low, indicating an insufficient capacity in predicting the prognosis of sepsis. The outcome showed the limitation of cortisol as an independent predictor.Cortisol was only a stress response to sepsis, so it was difficult to determine if cortisol could deal with infection attacks alone. The severity of infection should also be taken into consideration. P/C ratio was a relative ratio,including PCT. PCT is a good parameter to represent the severity of infection. Compared with IL-6, white blood cells, and other biomarkers, PCT is significantly associated with bacterial infection and sepsis shock. PCT is a more sensitive marker for evaluating the severity of sepsis in patients requiring corticosteroid treatment.[15]Therefore, P/C ratio was expected to have a potential predicting capacity for sepsis prognosis, which was confirmed in our study. In addition to a significant relationship with survival outcome (P＜0.001) and 28-day mortality (P=0.033), the AUC of P/C ratio was higher than that of PCT (0.919 vs. 0.876). RCS analysis also proved the good capability of P/C ratio in predicting 28-day mortality. However, few studies have reported the role of combined cortisol and PCT in predicting the prognosis of sepsis. Our results revealed that P/C ratio had a better predictive capability than PCT or cortisol alone, which helped understand the diagnosis of CIRCI.CIRCI is an interesting situation known as “starvation in plenty”. Corticosteroids increased significantly in critical illness but still not enough to meet the need for stress. As reported, 30% of critical patients received CIRCI, and the ratio increased to 50% for septic and septic shock patients.[16]Recent reports showed that CIRCI resulted in longer ICU length of stay, septic cardiomyopathy, and higher hospital mortality among patients with sepsis.[4,17]

The corticosteroid level rises rapidly during sepsis,which is the classical physiological process of the stress response. However, the absolute value of corticosteroids cannot represent adrenal function. The severity of infection is a crucial factor that should be considered.In the present study, the P/C ratio comprised two parts.One was cortisol, which represented the ability to deal with infection stress, and the other was PCT, which indicated infection severity. The total serum cortisol was examined in this study and was composed of free cortisol and protein-binding cortisol. Usually, 75%-80% cortisol is combined with corticosteroid-binding globulin (CBG), 15% cortisol is combined with serum albumin, and 5%-10% cortisol is free.[18,19]Combined and free cortisol can transfer to each other in a balance manner. Only free cortisol can enter the cytoplasm and initiate different pathophysiological processes. However,free cortisol measurement is not easy to perform in the clinic.[20]Therefore, total cortisol measurement was recommended by the CIRCI Guideline of 2017.[7]Thus, total serum cortisol was measured in the present study. PCT is a highly-recommended biomarker with reasonable specificity, that is predominately produced in the thyroid gland and liver, lungs, and intestines, and is elevated during acute tissue inflammation and tissue injury.[21]PCT levels differentiate bacterial infection from nonspecific inflammation.[22]A recent study approved by FDA showed that the PCT assay supported clinicians in making better decisions when dealing with an infection.It is a reliable tool for initiating and de-escalating antibiotic treatment.[23]Previous report also revealed that PCT is a highly specific sepsis biomarker, especially during abdominal infection.[24]Therefore, P/C ratio represents the systemic stress response capacity against infection.

However, there are several limitations of our study. This was a retrospective clinical cohort study conducted in a single center. Therefore, bias could not be avoided. Furthermore, the non-survival group had higher IL-6, SOFA, and APACHE II scores, which could be confounders. However, the results were confirmed by logistic regression analysis. The sample size was small, affecting the generalizability of the results.Therefore, a randomized control trial with a large sample size is needed to confirm our hypothesis. In addition, more clinical centers were required to be included. Intervention trials using prospective methods might be designed on the P/C ratio, which could help determine who is most likely to benefit from corticosteroid treatment.[25,26]

CONCLUSION

P/C ratio showed a good prognostic predicting capability in sepsis. In addition, P/C ratio helped assess the heterogeneity of CIRCI and could be a potential tool in corticosteroid replacement therapy.

Funding:Not applied.

Ethical approval:The authors are responsible for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The institutional ethics committee of the General Hospital of the People’s Liberation Army approved this singlecenter retrospective cohort study (No. S2017-054-01). The study was conducted in accordance with theDeclaration of Helsinki(as revised in 2013). Because of the retrospective nature of the research, the requirement for informed consent was waived.

Conflicts of interest:All authors have completed the ICMJE uniform disclosure form. The authors have no conflicts of interest to declare.

Contributors:conception and design: HL, FHZ; administrative support: JGX, HJK, FHZ; provision of study materials or patients:JGX, HJK, FHZ; collection and assembly of data: HL, JH; data analysis and interpretation: HL, JH, JGX; manuscript writing: all authors; final approval of manuscript: all authors.