Diagnostic value of urinary microprotein concentration for patients with negative urinary protein test results and positive urinary casts on microscopic examination

Abstract Objective To analyze the association between positive urinary casts on microscopic examination and urinary microprotein concentration in the case of negative urinary protein test results. This study also investigated the diagnostic value of urinary microprotein examination. Subjects A total of 949 samples that were analyzed with a UF‐1000i Urine Analyzer and returned cast alarm results were categorized into two groups, a positive and negative group, according to qualitative urinary protein sulfosalicylic acid test results. Then, 54 samples with negative protein test results but positive cast results according to microscopic examination were selected as the study group; 60 normal people with healthy physical examination results were selected as the control group. Both groups underwent urinary microprotein tests, including urinary microalbumin (mAlb), α1‐microglobulin (A1M), transferrin (TRU), and immunoglobulin G (IgG). T tests were used to evaluate mean differences between groups and chi‐square tests were used to calculate ratio differences between groups. Results (a) Microscopic examinations of the positive and negative protein groups revealed no statistically significant difference in cast detection rate (P = .421). (b) Among the 54 samples in the study group, 37 were found to have abnormal casts, while in the remaining 17 samples, only hyaline casts were detected. (c) The detection levels of mAlb, A1M, and IgG in the study group were significantly higher than the control group (P values < .05). Conclusion Urinary microprotein test should be included in the re‐examination rules for routine tests for patients with negative protein results and positive casts under microscopic examination.


| INTRODUC TI ON
With the increasing application of automatic urine analyzers, increased attention has been directed at improving the accuracy and efficiency of the test results. Research suggests that the results for erythrocytes, leukocytes, and epithelial cells obtained using automated analyzers are similar to the results of microscopic examinations. However, to eliminate error or uncertainty, some images (particularly those of dysmorphic cells, bacteria, yeasts, casts, and crystals) must be analyzed by manual microscopic examination. 1 In practice, due to the limitations of the instrumental test method and the complexity of the composition of urine, a situation often arises where the chemistry result does not match the formation test result, that is, there are cases of negative protein test results with observed pathologic casts. 2,3 The presence of urinary proteins is often associated with a high risk of renal and cardiovascular diseases such as nephrotic syndrome, chronic nephritis, lupus nephritis, hypertensive nephropathy, diabetic nephropathy, and so on. Traditionally, the formation of casts is closely related to the presence of urinary proteins. The presence of pathological casts, including granular cells, epithelial cells, red cells, white cells, and waxy casts in urine is a sensitive diagnostic indicator reflecting whether the kidney has suffered substantial damage; thus, the accuracy of detection of pathological casts is critical. 4 In the case of samples negative for urinary proteins, abnormal casts are often overlooked under microscopic examination. Hence, it is necessary to develop reasonable and effective re-examination rules to avoid miss examinations and false examinations.
These have important diagnostic value in diseases such as diabetic nephropathy, cardiovascular disease caused by rheumatoid arthritis, and hypertensive renal injury. [5][6][7][8] However, testing for these proteins is costly and time-consuming. Immunonephelometry for urinary microproteins is particularly costly.  an Olympus microscope is used to observe various components in urine (Olympus Corporation); Beckman Coulter IMMAGE 800 specific protein analyzer is based on a immunonephelometric method to detect the urinary microprotein, matching reagents, and Bio-Rad quality control materials (Bio-Rad Corporation), and sulfosalicylic acid reagent with a reagent concentration of 200 g/L. true-positive cast rates were determined for each group. In total, 54 samples negative for urinary proteins (validated by sulfosalicylic acid) were selected as the study group; 60 normal people with healthy physical examination results who were patients at our hospital during the same period of time were selected as the control group. mAlb, A1M, TRU, and IgG were tested using an IMMAGE 800 specific protein analyzer.

| Statistical methods
The research data were processed and analyzed using SPSS 22.0 statistical software. Measurement data are expressed as means ± standard deviations (x ± S); t tests were used to compare between groups. Count data are expressed as rates (%); chi-square tests were used to calculate differences between groups. P < .05 indicates a statistically significant difference.  Table 1.

| Analysis of casts in the study group
Among the 54 samples in the study group, 37 (68.52%) were found to have pathological casts; the remaining 17 (31.48%) only had hyaline casts.

| Analysis of clinical data in the study group
In the study group, 13 patients were diagnosed with kidney disease or disease that may cause secondary renal disease, such as diabetes and hypertension. Among them, 11 cases were found to have pathological casts and two cases were found to have hyaline casts.

| Comparison of the positive detection rates of the four urinary microproteins between the study group and control group
The positive rates of mAlb, A1M, TRU, and IgG of two group were shown in Table 2, respectively. The positive rates of the four urinary microproteins in the study group were significantly higher than those in the control group (P < .05).

| Comparison of detection levels of the four urinary microproteins between the study group and control group
The detection levels of mAlb, A1M, TRU, and IgG of two group were shown in Table 3. The results indicated the detection levels of mAlb, A1M, and IgG in the study group were significantly higher than the control group (all P < .05). The TRU level of the study group was slightly higher than that of the control group, but this difference was not statistically significant (P > .05).

| D ISCUSS I ON
Pathological casts are a very important part of urine sediment examinations and are the main indicator for determining whether there is pathological parenchymal damage of the kidney. 10 The traditional view is that the presence of casts in the urine is usually accompanied by positive protein test results. However, in practical daily work, it is uncommon for a urine analyzer to report abnormal cast with negative urinary protein results. In this study, we found that the rate of negative protein results by dry chemistry with the UF-1000i analyzer reached 31.71%, which is close to the rate published in a recent study (20.29%). 11 Our study indicated that the true-positive rates of the negative protein group and the positive protein group were  The diagnostic value of the four urinary microproteins in early kidney injury has been confirmed. In particular, a number of studies have highlighted the diagnostic significance of urinary mAlb for kidney disease. 5 decreased glomerular filtration. 17,18 Urinary IgG is also an important marker protein for early damage to the glomerular filtration membrane selective barrier and is positively correlated with glomerular damage. 19 In the current study, we also found that the positive rate of urinary microproteins mAlb, A1M, TRU, and IgG in the study group was significantly higher than that of the control group (P < .05). This indicates that the levels of these four urinary microproteins in patients with negative chemical urinary protein results and positive cast results were higher than those in healthy subjects. The above results suggest that a patient's kidneys may be damaged in the case of negative uri- Based on the current results, we can conclude the following: 1. Regardless of whether the qualitative urinary protein result is positive or negative, a UF-1000i urine analyzer cast alarm must be reexamined microscopically.
2. Urinary microprotein test results should be included in the reexamination rules for routine urine tests; this would improve the cast detection rate and would allow prediction of whether parenchymal damage to the kidney has occurred in the early stage.
Additional recheck rules can be identified and amended manually to obtain accurate and reliable results. Laboratories should develop re-examination rules that are applicable to their own circumstances and should continue to perform clinical validation and adjustment of these rules. This will greatly improve the accuracy of the test results as well as work efficiency.