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Self-inflicted injury by cutting and piercing instruments was the most common method in both groups, occurring in 30.8% of hospitalizations for major depressive disorder and 35.5% for bipolar disorder. The prevalence was significantly higher in bipolar disorder than in major depressive disorder (P < .001).

Compared with bipolar disorder, hospitalizations for major depressive disorder more often involved analgesics (16.7% vs 11.0%, P = .000), other sedatives and hypnotics (4.9% vs 3.7%, P < .001), other and unspecified solid and liquid substances (3.9% vs 3.3%, P = .037), and hanging by strangulation and suffocation (3.7% vs 2.5%, P < .001).

By contrast, cutting and piercing instruments were more common in bipolar disorder than in major depressive disorder (35.5% vs 30.8%, P < .001).

Yes. Use of tranquilizers and other psychotropic agents was common and nearly identical in the two groups, at 29.2% in bipolar disorder and 29.4% in major depressive disorder (P = .72). Other specified drugs and medicinal substances were also similar, at 14.3% in bipolar disorder and 14.0% in major depressive disorder (P = .546).

In this inpatient sample, anxiety disorders and adjustment disorders were more prevalent in major depressive disorder than in bipolar disorder. Anxiety disorders occurred in 41.7% of major depressive disorder admissions versus 39.7% of bipolar disorder admissions (OR = 0.920, CI 0.864-0.981, P = .011), and adjustment disorders occurred in 2.3% versus 1.2%, respectively (OR = 0.498, CI 0.386-0.644, P < .001).

Schizophrenia and other psychotic disorders and personality disorders were more common in bipolar disorder than in major depressive disorder in this study. Schizophrenia and other psychotic disorders were present in 6.1% of bipolar disorder admissions versus 2.6% of major depressive disorder admissions (OR = 2.486, CI 2.125-2.909, P = .000), and personality disorders were present in 27.5% versus 18.5% (OR = 1.670, CI 1.548-1.802, P = .000).

The study included adults aged 18 years or older who were admitted to the hospital with major depressive disorder or bipolar disorder, identified from discharge records in the National Inpatient Sample. The analysis used 2012-2015 data and included 13,556 unweighted records for major depressive disorder and 6,506 for bipolar disorder.

This was a retrospective analysis of the 2012-2015 National Inpatient Sample, a large US inpatient database representing a roughly 20% stratified sample of community hospital discharges. The findings describe associations and prevalence patterns in hospitalized adults with major depressive disorder and bipolar disorder, but they do not establish causation.

The authors also noted several limitations that affect interpretation: administrative data are subject to detection, reporting, and coding bias; coding errors could not be individually confirmed; and because the database is based on discharge records rather than unique patients, the same person may have been counted more than once.

Patients hospitalized with major depressive disorder were older on average than those hospitalized with bipolar disorder, with a mean age of 38.74 years versus 35.92 years. Both groups had a higher proportion of female patients (62.0% in major depressive disorder and 62.7% in bipolar disorder, P = .374) and White patients (73.9% and 77.7%, respectively, P < .001).

Admissions in both groups were most common from areas in the 0th to 25th percentile of median household income (27.6% for major depressive disorder and 31.2% for bipolar disorder, P < .001), and most patients in both groups were discharged home (90.6% in each group, P = .164).