Background: Systemic lupus erythematosus (SLE) is a chronic inflammatory autoimmune disease of unknown cause that presents with a wide range of clinical symptoms. Renal involvement in SLE often manifests as lupus nephritis and has become a leading cause of death among Asian populations. Proteinuria serves as a key indicator for assessing renal involvement, monitoring disease progression, and evaluating treatment outcomes in patients with SLE. Aim: The aim of this study was to evaluate the correlation between the spot albumin-to-creatinine ratio, protein-to-creatinine ratio, and 24-hour proteinuria in patients with systemic lupus erythematosus. Material and Methods: The study was conducted using a hospital-based retrospective research method and included 41 patients with SLE who attended the rheumatology outpatient clinic at the Mongolian-Japanese Hospital from July 2024 to February 2025. The participants were divided into two groups based on the level of proteinuria in their 24-hour urine sample: Group A (more than 140 mg) and Group B (less than or equal to 140 mg). To prevent errors in the results of the 24-hour urine protein test, participants were provided with instructions on proper specimen collection prior to the test. Statistical analysis was performed using SPSS version 30.0, and correlations were assessed using Spearman’s rank correlation coefficient. Results were considered statistically significant at p<0.001. Results: A total of 41 participants were included in the study: 56.1% (n=23) in Group A and 43.9% (n=18) in Group B. The mean age of the participants was 37±9.74 years, with 92.7% (n=38) being female and 7.3% (n=3) being male. The mean 24-hour urine protein loss was 1883.1±1819.5 mg/24h in Group A and 72.8±29.0 mg/24h in Group B. The normal urine albumin-to-creatinine ratio (ACR) is less than 30 mg/g. In Group A, 8.7% (n=2) had a normal ACR, while 91.3% (n=21) had abnormal results. Normally, the urine protein-to-creatinine ratio (PCR) is less than 0.15 g/g. In Group A, 13% (n=3) had a normal PCR, while 87% (n=20) had abnormal results. In Group B, 94.4% (n=17) had an ACR <30 mg/g, and 5.6% (n=1) had an ACR >30 mg/g, with all participants in this group having a normal PCR. Correlation analysis between 24-hour urine protein and the albumin-to-creatinine ratio (ACR) showed a strong positive correlation (r=0.790, p<0.001). Similarly, the correlation between 24-hour urine protein and the protein-to-creatinine ratio (PCR) demonstrated a high correlation (r=0.829, p<0.001), indicating a statistically significant relationship. Conclusion A total of 41 participants with SLE were included in the study, with 56.1% (n=23) exhibiting proteinuria. We have shown that the spot urine protein-to-creatinine ratio and albumin-to-creatinine ratio are highly correlated with the 24-hour urine protein measurement. Therefore, these tests are suggested as practical adjuncts to the 24-hour urine protein measurement for identifying urinary protein in patients with SLE.