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Background: Total joint arthroplasty (TJA) is increasingly being performed as an outpatient (i.e., same-day discharge) procedure. Postoperatively, orthostatic hypotension or pain-related tachycardia can lead to concerns regarding the hemoglobin (Hgb) level of the patient prior to discharge. The purpose of this study was to prospectively assess the reliability and accuracy of, and patient and nurse satisfaction with, postoperative noninvasive hemoglobin (nHgb) monitoring compared with an invasive serum hemoglobin (iHgb) laboratory draw in the outpatient TJA setting. Methods: We prospectively enrolled 200 patients undergoing outpatient unilateral TJA, of whom 157 were ultimately included in our analysis (94 hips, 63 knees). Postoperatively, both nHgb and iHgb values were obtained at a mean of 36 minutes apart. Surveys were completed by patients and nurses. The strength of the agreement between the 2 Hgb monitoring methods was evaluated with use of the Bland-Altman 95% limits of agreement, concordance correlation coefficient (CCC), and intraclass correlation coefficient (ICC). Receiver operating characteristic curve analyses were performed to investigate the ability of nHgb monitoring to predict an iHgb of <11.2 g/dL (the 5th-percentile iHgb value). Results: The mean preoperative iHgb was 14.2 ± 1.1 g/dL. The mean postoperative iHgb and nHgb values were 13.3 ± 1.5 and 13.3 ± 1.2 g/dL, respectively. The Bland-Altman 95% limits of agreement were −3.2 and +3.1 g/dL, indicating that 95% of patients' iHgb values are expected to fall between these limits relative to the nHgb value of the patient. The CCC and ICC were both 0.33. An nHgb cutoff value of <12.7 g/dL had 100% sensitivity and 67% specificity for detecting an iHgb of <11.2 g/dL. Patients reported less pain with the nHgb test than with the iHgb test (mean pain score, 0.0 versus 1.8; p < 0.001), and 97% of patients preferred the nHgb test. Following the nHgb test, 73% of responding patients and 83% of responding nurses were "somewhat more reassured" to "significantly more reassured" about same-day discharge. Conclusions: Routine nHgb testing can rapidly screen patients undergoing outpatient TJA for acute anemia prior to discharge. With an nHgb of <12.7 g/dL, there was perfect sensitivity and 67% specificity for detecting an iHgb of <11.2 g/dL. Most patients and nurses felt more reassured about same-day discharge after nHgb monitoring. Level of Evidence: Diagnostic Level II. See Instructions for Authors for a complete description of levels of evidence. [ABSTRACT FROM AUTHOR]