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Background: The clinical significance of a trace protein reading on urinalysis is unclear, and such a result is often ignored by the clinician. Methods: We examined 185 samples of urine with trace proteinuria by both Chemstrips and sulfosalicylic acid testing, and compared the results with those of urinary albumin and total protein concentrations. Results: Taking for the purposes of this study an arbitrary upper limit of normal of 20 mg/l for albumin and 100 mg/l for total protein concentration, we found abnormal albumin excretion in 87% and abnormal total protein excretion in 88% of trace samples. In this study, a negative urinalysis for protein excluded microalbuminuria in 87% and proteinuria in 78% of cases. Conclusion: Qualitative testing for protein by urinalysis has a high sensitivity and specificity for diagnosing or ruling out microalbuminuria. Trace proteinuria usually means microalbuminuria; negative proteinuria tends to rule it out.

Carbamylated proteins formed in renal insufficiency from the spontaneous decomposition of urea exert a variety of metabolic effects. Here we examined the effects of carbamylated proteins on glomerular mesangial cells to determine whether urea retention in early renal insufficiency may itself promote glomerular sclerosis and hasten the progression to kidney failure. To this effect we carbamylated fetal bovine serum proteins in vitro and tested their effect on mesangial cell proliferation (by tritiated thymidine uptake), de novo protein synthesis (by tritiated leucine uptake), collagen I and collagen IV accumulation (by avidin-biotin enzyme immunoassay), and gelatinase levels in the medium (by zymography and quantitative fluorescence assay). Carbamylated fetal bovine serum at concentrations present in uremia increased tritiated thymidine incorporation by 50% without altering tritiated leucine incorporation, and it increased collagens I and IV in the monolayer by 150% to 300%. Gelatinase activity was unchanged. We conclude that carbamylated proteins can activate mesangial cells to a profibrogenic phenotype. From a clinical perspective, the carbamylation of proteins by elevated urea levels may accelerate the progression to kidney failure and thus set up a vicious cycle in which the nitrogen retention itself would cause further progression of fibrosis and deterioration of kidney function.

We re-addressed the question of whether routine total urinary protein determinations can be used to predict the presence of microalbuminuria by studying 61 patients who attended a diabetic clinic and tested negative or had one positive protein by dipstick. Total urinary protein was measured by the Biorad dye-binding method in undialyzed urine (UND), in dialyzed urine (DIAL), and in dialyzed urine in which albumin and globulins were separated, measured separately with albumin and globulin standards and the results added together to obtain total urinary protein (A + G). The results were compared with albumin measurements obtained by radioimmunoassay (RIA). Compared to DIAL, urinary protein measurements were 43% higher with A + G and 22% higher with UND. Microalbuminuria correlated moderately with UND (r = 0.81) and better with the other methods (r = 0.87 for DIAL, r = 0.91 for A + G). None of the methods predicted microalbuminuria reliably. Taking a protein-to-creatinine ratio of 0.15 and an albumin-to-creatinine ratio of 0.03 as upper limits of normal, we found that UND had a 72% positive predictive value (28% false positives) and 85% negative predictive value (15% false negatives). DIAL had 90% positive predictive value (10% false positives) and 78% negative predictive value (22% false negatives). A + G had 65% positive predictive value (35% false positives) but 91% negative predictive value (9% false negatives). A + G, which uses the correct standards, would be the most suitable method for screening, having the least number of false negatives, but has more false positives because it is more sensitive. In practice, most routine chemical laboratories find it expedient to use only UND, but physicians interpreting the results of this method should be aware of its limitations.

We have previously found that carbamylated hemoglobin (carHb) levels are increased in chronic renal failure and correlate positively with blood urea nitrogen (BUN) levels and with the duration of exposure to urea. In a fashion analogous to glycosylated hemoglobin in diabetic patients, it is possible that carHb may better reflect BUN levels before hemodialysis (preBUN) and also between hemodialysis sessions. We therefore tested the hypothesis that carHb could be a better index of adequacy of hemodialysis than the urea reduction ratio (URR). Fifty hemodialysis patients had carHb measured every 2 months for 14 months; the carHb level was compared with URR and preBUN levels, as assessed by changes in absolute numbers and trends of the BUN levels between hemodialyses. Mean URR was above 61% throughout the 14 months. Mean carHb levels did not change significantly during the study and were only weakly correlated with URR. However, there was a much better correlation between predialysis BUN and carHb, suggesting that carHb levels reflect more accurately the changes in BUN between hemodialysis sessions. To further test this hypothesis, we subdivided the patients arbitrarily, depending on the change in preBUN between two consecutive carHb measurements. We found significantly lower carHb levels when BUN decreased or remained stable than when it increased or was persistently high. In patients with decreasing or stable BUN, carHb was significantly lower than in patients with persistently high or increasing BUN (carHb 81.5 +/- 3.6 microg valine hydantoin [VH]/g Hb v 123.7 +/- 11.7 microg VH/g Hb, respectively; P0.001). URR was not different between groups. In addition to changes in BUN levels, carHb was correlated by multiple regression analysis with the presence of diabetes, weight, and plasma HCO3. The relationship between diabetic patients and carHb levels was complex because such patients tend to have higher preBUN levels, higher protein catabolic rate, and lower HCO3 levels. These results demonstrate that carHb reflects the changes between dialysis BUN and may serve as a more accurate index of uremia control. Clinically, it appears that well-dialyzed patients have carHb levels lower than 100 microg VH/g Hb.

Factors determining hemoglobin carbamylation in renal failure. Carbamylated hemoglobin (carhb) is formed by the reaction of hemoglobin with cyanate, a product of in vivo urea dissociation. It is found in high levels in patients with renal failure and may be useful in their clinical evaluation. Accordingly, we measured carhb by HPLC after acid hydrolysis in 73 patients with renal failure and 11 controls. Mean carhb levels, (expressed as μg valine hydantoin/g Hb), were highest in chronic renal failure (CRF, 146 ± 13), intermediate in end-stage renal disease on hemodialysis (ESRD, 106 ± 7), and lowest in acute renal failure (ARF, 80 ± 12) when compared to normal subjects (27 ± 2). In all patients carhb was significantly correlated with BUN but not with creatinine, bicarbonate, or phosphate. For any level of BUN above 80 mg/dl, carhb was substantially higher in CRF than in ARF. Predialysis BUN and urea reduction ratio (URR) were significant predictors of carhb in ESRD. To investigate the effect of time of exposure and BUN level on the rate of carbamylation of hemoglobin, blood from normal subjects and dialysis patients was incubated in vitro with urea equivalent to BUN levels of 50, 100, 150, and 200 mg/dl and assayed for carhb at 0, 5, 9, and 14 days. Carhb increased linearly over the first nine days of urea exposure and leveled off thereafter. The rate of carbamylation increased as BUN increased and was significantly higher in hemoglobin from dialysis patients than from normal subjects. These results show that the higher the level of carhb at baseline, the higher the rate of carbamylation upon exposure to increasing urea concentrations. We conclude that carhb formation is dependent on urea concentration and length of exposure to urea. The rate of carhb formation for a given urea concentration is greater in hemoglobin already carbamylated, and this may explain why carhb is higher in CRF than in ARF at BUN levels greater than 80 mg/dl. Carhb may thus be a useful index of the duration and degree of exposure to high blood urea levels in patients with renal failure, and may potentially serve as an index of the adequacy of dialysis.

Publisher Summary This chapter discusses the biochemistry of the uremic syndrome. In recent years, the ability to treat uremia has outstripped the understanding of its pathogenesis. A precise understanding of the pathogenesis of the uremic syndrome, however, continues to elude people. Numerous substances have been isolated from the blood of uremic subjects and have been studied in vivo and in vitro . Increasingly clinicians appreciate the role of acidosis, hyponatremia and water intoxication, anemia, hyperkalemia, hypocalcemia, hyperphosphatemia, hypertension, and heart failure. Clinicians rely mainly on blood urea nitrogen (BUN) and serum creatinine measurements to evaluate patients with renal failure. The metabolic changes—retention of a legion of substances; the loss of vital renal hormones (e.g., erythropoietin) and enzymes (e.g., 1α-hydroxylase); the effect of the uremic environment on organ function, intermediary metabolism, and transport processes; dialysis-related problems; and exogenous toxins—observed in uremia result from the decline in renal excretory function. The chapter focuses on the retention of inorganic substances such as hyperkalemia, hydrogen ion and trace elements, and organic substances including oxalate, urea, guanidine, and polyamines. The chapter concludes with a discussion of dialysis related problems and related drugs and exogenous toxins.

To further define the platelet abnormality responsible for uremic bleeding, we studied platelet aggregation with adenosine diphosphate, ristocetin, and collagen in serum fractions obtained by Sephadex G-15 chromatography. We found that uremic patients had considerable inhibition in several peaks of middle molecular range, but the findings were inconsistent and not clearly related to the degree of uremia.