Performance of principal scores to estimate the marginal compliers causal effect of an intervention

On-treatment serum albumin level can guide long-term treatment in patients with cirrhosis and uncomplicated ascites

BACKGROUND & AIMS

The ANSWER study reported that long-term albumin administration in patients with cirrhosis and uncomplicated ascites improves survival. During treatment, serum albumin increased within a month and remained stable thereafter. In this post hoc analysis, we aimed to determine whether on-treatment serum albumin levels could guide therapy.

METHODS

Logistic regression was used to assess the association between baseline serum albumin and mortality, as well as to determine on-treatment factors associated with mortality and to predict the achievement of a given on-treatment serum albumin level. Survival was assessed by Kaplan-Meier estimates and second-order polynomial regression. Patients whose on-treatment serum albumin remained below normal were compared with a subset of patients from the control arm matched by principal score.

RESULTS

Baseline serum albumin was closely associated with 18-month mortality in untreated patients; albumin treatment almost effaced this relationship. On-treatment serum albumin and MELD-Na at month 1 were the sole independent variables associated with mortality. Second-order polynomial regression revealed that survival improved in parallel with increased 1-month on-treatment serum albumin. Kaplan-Meier estimations showed that any value of 1-month on-treatment serum albumin (0.1 g/dl intervals) in the range 2.5-4.5 g/dl discriminated patient survival. In the normal range of serum albumin, the best discriminant value was 4.0 g/dl. Compared to untreated patients, survival even improved in patients whose on-treatment serum albumin remained below normal.

CONCLUSION

Baseline serum albumin per se should not guide the decision to start albumin therapy. Conversely, 1-month on-treatment serum albumin levels are strongly associated with outcomes and could guide the use of albumin - 4.0 g/dl being the target threshold. However, even patients whose serum albumin remains below normal benefit from long-term albumin administration.

LAY SUMMARY

The ANSWER study has shown that long-term albumin administration improves survival and prevents the occurrence of major complications in patients with cirrhosis and ascites. This study shows that the achievement of these beneficial effects is related to a significant increase in serum albumin concentration. Even though the best results follow the achievement of a serum albumin concentration of 4 g/dl, a survival benefit is also achieved in patients who fail to normalise serum albumin.

Identifying treatment responders using counterfactual modeling and potential outcomes

Individualizing treatment according to patients' characteristics is central for personalized or precision medicine. There has been considerable recent research in developing statistical methods to determine optimal personalized treatment strategies by modeling the outcome of patients according to relevant covariates under each of the alternative treatments, and then relying on so-called predicted individual treatment effects. In this paper, we use potential outcomes and principal stratification frameworks and develop a multinomial model for left and right-censored data to estimate the probability that a patient is a responder given a set of baseline covariates. The model can apply to RCT or observational study data. This method is based on the monotonicity assumption, which implies that no patients would respond to the control treatment but not to the experimental one. We conduct a simulation study to evaluate the properties of the proposed estimation method. Results showed that the predictions of the probability of being a responder were well calibrated even if we observed variability and a small bias when many parameters were estimated. We finally applied the method to a cohort study on the selection of patients for additional radiotherapy after resection of a soft-tissue sarcoma.