Over the study period we identified 703,484 patients who met the study eligibility criteria and initiated lipid-lowering therapy with statins, ezetimibe, or a combination of both. Among those, 68% had a recorded charge for at least one of the 23 study lab tests in the 6 months before treatment (Table ). This proportion increased to 72% if the covariate assessment period was extended to 9 months before treatment, and to 74% during a 12-month period. For patients with diabetes or RA the proportions were higher (80% during 6 months) but showed equally small increases if the covariate assessment period was extended (83% and 84%). For LDL and HbA1c tests, the proportion of patients with a recorded charge for at least one test during the 6 months before initiation of lipid-lowering therapy was about 60% and 17%, respectively. For patients with diagnosed diabetes, 68% had a charge for an HbA1c test (Table ).
Number of patients with at least one lab test performed and claimed among 703,484 initiators of statins and/or ezetimibe using 6, 9, and 12-month confounder assessment periods (CAPs)
Overall and regardless of having a test performed, the proportion of patients with any outpatient lab test results available in the linked database was about 30%, which was similar in patients with diabetes or RA. Lab test results for LDL or HDL were available for about 20% of patients during the 6 months before initiation of lipid-lowering therapy.
Table shows whether any of 23 outpatient lab tests, including LDL, HDL and HbA1c were performed within the 6 months before initiating lipid-lowering therapy cross-tabulated by a range of patient and health system characteristics. Overall, 481,133 (68%) of study patients had claims evidence of an outpatient lab test and 42% thereof had results available in the study data (29% of all patients). The proportion with at least one lab test performed varied substantially by patient characteristics, while test result availability varied little, and only for variables such as system characteristics and state of residence (Table ).
Patients with lab test results reported in study population of 703,484 initiators of statins or ezetimibe using a 6-month covariate assessment period*
Having been hospitalized in the 30 days before the initiation of lipid-lowering treatment was negatively associated with receiving an outpatient test, likely because the relevant lab tests were performed during the hospitalization and as such do not appear as outpatient lab tests. Some patients hospitalized for acute coronary syndrome or MI may have received lipid-lowering therapy for secondary prevention without the need for a lab test. This is supported by the fact that patients with both recent MI and ACS had a lower than average proportion with at least one test performed (24% and 40% compared with an average of 68%). A code for hypercholesterolemia is frequently accompanied by an LDL test performed (80%) likely because the test ordering is accompanied with such a billing code.
Patients with Medicare Supplemental coverage (43,645) had a much lower proportion of claims for LDL tests performed (18%), and of those only 2% had results available. The lab test provider may not have included the secondary payer on the claim.
The two lab test providers that provided data to the insurer do not operate in some states; for example, the availability of lab test results in one state was as low as 2% for LDL. Such low recording would not be dependent directly on patient characteristics as it affects an entire state and is driven by factors other than health status, though clinically relevant patient characteristics have varying prevalences across states.
Some patients resided in states not primarily covered by the health plan studied, and are covered only via accounts for nationally operating businesses (e.g., if the employer is based in another state, all employees may be members of a health plan in that other state, rather than the state of residence). For these patients, the availability of LDL test results is less than 10%. One state (#12) stands out as having a small proportion of patients with an outpatient LDL test performed (24%), but a much larger proportion of patients have a result available (51%). In this state, a larger proportion of providers are under HMO capitation agreements. Within these plans, under-recording of tests performed may be the result of bundled payment arrangements; however, results are still forwarded by the lab test providers resulting in the paradox of having more lab test results available in our database than performed as recorded in claims data. Among patients with Diabetes or RA, we found fundamentally similar results. Among elderly patients lab test results were more likely to be available among Medicare advantage enrollees than those patients covered through Medicare supplemental insurance (Table ).
Cross-tabulation between states and Medicare Advantage and Medicare Supplement status
Based on patient and system characteristics plus exposure and outcome status it was possible to predict with high sensitivity (97%) and specificity (94%) whether outpatient lab tests were performed in the 6 months before treatment initiation. The corresponding model c-statistics of the logistic regression models were between 0.89 and 0.93 (Figure ), indicating a very high predictive capacity. Strong independent associates of having an outpatient LDL test performed were a diagnosis of hypercholesterolemia or obesity, and carotid revascularization. Associates of low probability of doing LDL lab tests were recent hospitalization and being diagnosed with RA. Being older than 65 also decreased the chance of an LDL lab test, likely because of test underreporting due to bundled payments. Initiating high-intensity lipid-lowering treatment and dying in the study follow-up period were correlates of not having an outpatient LDL, HDL, or HBA1c test performed. Not surprisingly, the strongest predictor of having an HBA1c test performed was a diagnosis of diabetes or pre-diabetes.
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)
Among the patients for whom LDL, HDL, or HBA1c test levels were available, we then attempted to predict the actual lab levels based on their recorded patient and system characteristics. Using all observed factors described above, 17% of the variation could be explained (Figure ). Young age was the strongest correlate of increased high LDL (+20 mg/dl) and HbA1c (+0.5%) levels, suggesting that in younger age initiation of lipid-lowering therapy was more driven by lab test results, i.e. primary prevention, while in older age past coronary events and other risk factors were the triggers for statin initiation despite lower LDL levels (−17 mg/dl).
Correlates of selected lab test results among patients with lab test results available (darker means stronger correlations)
Higher intensity of lipid lowering treatment generally was correlated with a lower proportion of outpatient LDL tests performed, a lower fraction of LDL test results available in the database, and lower LDL serum levels (Table ). For example, among high dose simvastatin initiators (>40 mg/day), 52% had an outpatient LDL test performed before treatment start (63% for lower dose simvastatin). Of those patients, 37% had a test result available (42%), and the mean LDL serum level was 135.6 mg/dl compared to 147.3 mg/dl for patients started on low-intensity simvastatin. Mean LDL levels were generally lower in patients with diabetes who initiated lipid-lowering therapy.
LDL tests performed and LDL test results available by lipid lowering treatment in patients initiating lipid-lowering therapy, including patient subgroups with diabetes (DM) or rheumatoid arthritis (RA)