Supplementing claims data with outpatient laboratory test results to improve confounding adjustment in effectiveness studies of lipid-lowering treatments

Abstract

Background: Adjusting for laboratory test results may result in better confounding control when added to administrative claims data in the study of treatment effects. However, missing values can arise through several mechanisms.

Methods: We studied the relationship between availability of outpatient lab test results, lab values, and patient and system characteristics in a large healthcare database using LDL, HDL, and HbA1c in a cohort of initiators of statins or Vytorin (ezetimibe & simvastatin) as examples.

Results: Among 703,484 patients 68% had at least one lab test performed in the 6 months before treatment. Performing an LDL test was negatively associated with several patient characteristics, including recent hospitalization (OR = 0.32, 95% CI: 0.29-0.34), MI (OR = 0.77, 95% CI: 0.69-0.85), or carotid revascularization (OR = 0.37, 95% CI: 0.25-0.53). Patient demographics, diagnoses, and procedures predicted well who would have a lab test performed (AUC = 0.89 to 0.93). Among those with test results available claims data explained only 14% of variation.

Conclusions: In a claims database linked with outpatient lab test results, we found that lab tests are performed selectively corresponding to current treatment guidelines. Poor ability to predict lab values and the high proportion of missingness reduces the added value of lab tests for effectiveness research in this setting.

Figure 1

Reasons for missing lab test results in a longitudinal healthcare utilization database linked to a lab test provider database*. * In the setting of a new user cohort study with a defined covariate assessment period before the first exposure and before follow-up

Figure 2

Incident user cohort study*. * The 6-month covariate assessment period (CAP) precedes the initiation of treatment. During the CAP we identified patient characteristics, including lab tests performed and lab test results available. After treatment start followed a 1-month lag period before events were attributed to the treatment. The arrows between prescriptions (Rx), diagnoses (Dx) and lab tests denote the fact that the temporality of events within the CAP was not considered in this study

Figure 3

Associates of selected outpatient lab tests performed in patients initiating lipid-lowering treatment according to claims data in 703,484 patients from a logistic regression model (darker means stronger association)

Figure 4

Correlates of selected lab test results among patients with lab test results available (darker means stronger correlations)

Figure 5

Ability of longitudinal claims data to predict whether a lab test was performed, a test result was available, and the actual serum level for three biomarkers of cardiovascular risk*. * c-statistics were computed from multivariate logistic regression models including patient factors measured during 6 months before lipid-lowering treatment initiation; r² measured were computed only among patients who had a lab test result available from linear regression including patient factors measured during 6 months before lipid-lowering treatment initiation, actual treatment choice, as well as cardiovascular events and death during follow-up