Population characteristics
The characteristics of the 2019 total population and 10 year continuously insured cohorts for MarketScan (N = 16,365,997 and n = 951,173, respectively) and Medicaid (N = 4,414,153 and n = 785,088, respectively) are summarized in Table 1. The mean age (±SD) was older in MarketScan (38.1 ± 20.2 years) than in Medicaid (33.5 ± 24.3 years).
The proportion of females was similar in both databases (MarketScan 54.14%, 95% CI: 54.12–54.17; Medicaid 55.68%, 95% CI: 55.64–55.73), and increased in the 10 year insured cohort (MarketScan 62.02%, 95% CI: 61.92–62.12; Medicaid 61.11%, 95% CI: 61.10–61.16). In the Medicaid 2019 total population, the Multiracial group was the largest (31.53%, 95% CI: 31.49–31.58), followed by White (26.45%, 95% CI: 26.41–26.49), Black (15.78%, 95% CI: 15.75–15.81), Hispanic or Latinx (12.17%, 95% CI: 12.14–12.20), Asian (9.63%, 95% CI: 9.60–9.66), and Native American (0.31%, 95% CI: 0.30–0.31) race and ethnicity groups. In comparison, in the Medicaid 10 year insured cohort the White group was the largest (31.19%, 95% CI: 31.11–31.30), followed by Multiracial (21.83%, 95% CI: 21.74–21.92), Hispanic or Latinx (19.10%, 95% CI: 19.01–19.19), Black (15.76%, 95% CI: 15.68–15.84), Asian (9.63%, 95% CI: 9.60–9.69), and Native American (0.29%, 95% CI: 0.28–0.30) race and ethnicity groups.
Prevalence
The prevalence of schizophrenia in 2019 for the total MarketScan and Medicaid populations and the 10 year continuously insured cohorts is summarized in Table 2. The prevalence of schizophrenia in the total population was higher in Medicaid (2.13%, 95% CI: 2.12–2.15; n = 94,153 of 4,414,153) than in MarketScan (0.134%, 95% CI: 0.132–0.135; n = 21,963 of 16,365,997). In both databases the prevalence was higher among the subsample of individuals with 10 years of continuous insurance prior to 2019, but this increase was more marked in Medicaid (5.74%, 95% CI: 5.69–5.79 vs. 2.13%) than in MarketScan (0.159%, 95% CI: 0.152–0.168 vs. 0.13%). Males had a higher prevalence than females (Medicaid: 2.80%, 95% CI: 2.78–2.83 vs. 1.60%, 95% CI: 1.58–1.62; MarketScan: 0.135%, 95% CI: 0.133–0.138 vs. 0.133%, 95% CI: 0.131–0.136) in the total population, as well as in the 10 year continuously insured cohort (Medicaid: 7.89%, 95% CI: 7.80–7.99 vs. 4.37%, 95% CI: 4.31–4.43; MarketScan: 0.183%, 95% CI: 0.170–0.198 vs. 0.145%, 95% CI: 0.136–0.155).
The prevalence of schizophrenia by age group for the total population and 10 year continuously insured cohort is presented in Fig. 2. The prevalence of schizophrenia in the Medicaid population increased over the course of the lifespan up to age 56–60 years (total population: 5.0%; 10 year continuously insured cohort; 11.3%). However, in MarketScan the prevalence peaks among young adults (0.25% for the total population at 21–25 years old, and 0.78% among those with 10 years of continuous insurance at 26–30 years old).
We examined the prevalence of schizophrenia in 2019, by age group, in the total population, and in the 10-year cohort (the subset of the total population with 10 years of continuous insurance prior to 2019) in the Medicaid and MarketScan study populations. In 2019, and for each year of the 10-year cohort, individuals were required to have continuous insurance with a maximum gap of 45 days per year, and one or more health service per year to ensure an opportunity to be observed. Prevalent cases were defined as those with two or more diagnoses of schizophrenia or schizoaffective disorder (ICD-9: 295.x; ICD-10: F20, F25; excluding schizophreniform: 295.4x, F20.81) in 2019.
Incidence
The incidence of schizophrenia diagnoses in 2019 for the 10 year continuously insured cohort is presented in Table 3. Incidence was higher in Medicaid (0.19%, 95% CI: 0.18–0.20; n = 1,489 newly diagnosed in 2019); than in MarketScan (0.07%, 95% CI: 0.07–0.08; n = 712), and for males than females (Medicaid: 0.21%, 95% CI: 0.20–0.23 vs. 0.18%, 95% CI: 0.16–0.19; MarketScan: 0.08%, 95% CI: 0.07–0.09 vs. 0.07%, 95% CI: 0.07–0.08). While incident cases made up only a small proportion of the prevalent cases in the 10 year continuously insured cohort in Medicaid in 2019 (3.3%, 95% CI: 3.1–3.5), incident cases constituted nearly half of prevalent cases in MarketScan (46.9%, 95% CI: 44.4–49.4).
Incidence by age group is presented in Fig. 3. The pattern is similar in both databases, with incidence rising rapidly in late adolescence and peaking in young adulthood (26–30 years of age, Medicaid: 0.40%; MarketScan: 0.16%). New cases of schizophrenia continue to emerge over the lifespan in both populations, but at a lower frequency.
We examined new onset (incidence) of schizophrenia in 2019 among individuals with 10 years of continuous insurance prior to 2019 in Medicaid and MarketScan. New onset of schizophrenia in 2019 was defined as those with two or more diagnoses of schizophrenia or schizoaffective disorder in 2019 (ICD-9: 295.x; ICD-10: F20, F25; excluding schizophreniform: 295.4x, F20.81), and no diagnoses of schizophrenia observed in the 10 years prior to 2019. Individuals were required to have continuous insurance with a maximum gap in insurance of 45 days per year, and one or more health services per year, during 2019 and each of the 10 years prior, to ensure an opportunity to observe a prior diagnosis of schizophrenia.
Accuracy of incidence measures
Table 4 summarizes the positive predictive value (PPV) for schizophrenia incidence measures with different periods of observation prior to the first schizophrenia diagnosis in 2019, using a 10 year reference standard. One year of observation prior to the index schizophrenia diagnosis yielded a PPV of 30% (95% CI: 29–31) for the total Medicaid population, and 51% (95% CI: 48–55) for those ≤35 years of age. The one year prior period incidence measure performed better in MarketScan, with a PPV of 81% (95% CI: 79–84) in the total MarketScan population, and 79% (95% CI: 73–84) for those ≤35 years of age. Increasing the period of observation prior to the index schizophrenia diagnosis increased the PPVs of the incidence measures in both databases. A PPV of ≥95% (96%, 95% CI: 93–97) was achieved in Medicaid for individuals ≤35 years of age using a minimum of seven years of prior observation, while for MarketScan a minimum period of only two years of prior observation (95%, 95% CI: 90–97) was required. For both Medicaid and MarketScan, PPVs of the incidence measures tested were generally higher for females than for males.
Figure 4 presents the PPVs of schizophrenia incidence measures by age group and by years of observation prior to the index schizophrenia diagnosis in 2019. In general, PPVs for new onset schizophrenia measures increase with increasing years of observation prior to the index diagnosis, and are higher for younger age groups. In Medicaid, incidence measures had the highest PPVs for individuals 11–15 years of age, followed by 16–20 years, 21–25 years, 26–30 years, and 31–35 years, with the lowest for individuals 65+ years of age. In MarketScan, the PPVs for incident measures for all age groups were generally higher than Medicaid for all age groups, but the younger age groups performed relatively less well, with the lowest PPV for individuals 26–30 years of age who achieved a 91% PPV by three years, and required seven years of prior observation to achieve a ≥95% PPV.
We examine the positive predictive value of measures of new onset schizophrenia among individuals that were identified as having schizophrenia in 2019, and who had 10 years or more of continuous observation prior to 2019 in Medicaid or MarketScan. We test the positive predictive value of alternative measures of new onset schizophrenia using sequentially longer lookback periods “x” (1, 2, 3, 4, 5, 6, 7, 8, and 9 years), compared to our reference standard for new onset schizophrenia (10 years lookback with no prior evidence of schizophrenia). The Positive Predictive Value = True Test Positive/ Test Positive cases. True Positive tests are those cases where using a lookback period of “x” provided an accurate result (consistent with the reference standard using a 10-yearlookback period). Test Positive cases are those defined as having new onset of schizophrenia using a specific lookback period of “x” years prior to 2019 (no prior schizophrenia diagnosis observed in “x” years lookback period).
The specificity of measures of new onset schizophrenia was high in both Medicaid and MarketScan, with ≥95% specificity achieved in both databases using two or more years of observation prior to the index schizophrenia diagnosis (Table 5). Specificity was generally higher in Medicaid than in MarketScan (92.10%, 95% CI: 91.84–92.35 vs. 79.93%, 95% CI: 76.99–82.64 for the total population using one year of observation prior to the index schizophrenia diagnosis).
