BACKGROUND: Registry-based studies of nephrotic syndrome (NS) may only include a subset of patients with biochemical features of NS. To address this, we compared patients with laboratory-recorded nephrotic proteinuria and hypoalbuminemia to patients with hospital-recorded NS. METHODS: We identified adult patients with first-time hospital-recorded NS (inpatients, outpatients, or emergency-room visitors) in the Danish National Patient Registry and compared them with adults with first-time recorded nephrotic proteinuria and hypoalbuminemia in Danish laboratory databases during 2004-2018, defining the date of admission or laboratory findings as the index date. We characterized these cohorts by demographics, comorbidity, medication use, and laboratory and histopathologic findings. RESULTS: We identified 1139 patients with hospital-recorded NS and 5268 patients with nephrotic proteinuria and hypoalbuminemia; of these, 760 patients were identified in both cohorts. Within 1 year of the first record of nephrotic proteinuria and hypoalbuminemia, 18% had recorded hospital diagnoses indicating the presence of NS, and 87% had diagnoses reflecting any kind of nephropathy. Among patients identified with nephrotic proteinuria and hypoalbuminemia, their most recent eGFR was substantially lower (median of 35 versus 61 ml/min per 1.73 m2), fewer underwent kidney biopsies around the index date (34% versus 61%), and the prevalence of thromboembolic disease (25% versus 17%) and diabetes (39% versus 18%) was higher when compared with patients with hospital-recorded NS. CONCLUSIONS: Patients with nephrotic proteinuria and hypoalbuminemia are five-fold more common than patients with hospital-recorded NS, and they have a lower eGFR and more comorbidities. Selective and incomplete recording of NS may be an important issue when designing and interpreting studies of risks and prognosis of NS.