Low molecular weight proteins in children with renal disease

Analysis of the ratio of urinary beta-2-microglobulin to total protein concentration in children with isolated tubulointerstitial disease

BACKGROUND

Proteinuria is broadly classified into glomerular and tubular proteinuria. Urinary beta-2-microglobulin (β2-MG) is known as a marker for detecting tubulointerstitial diseases. However, tubulointerstitial damage can also lead to an increase in urinary β2-MG level in some patients with glomerular diseases. This study aimed to determine the ratio of urinary β2-MG to total protein (TP) concentration in patients with both isolated tubulointerstitial and glomerular disease.

METHODS

This multicenter, retrospective study included children with Dent disease or lupus nephritis in five facilities. Their urinary β2-MG levels were > 1000 µg/L. Urinary β2-MG and TP concentrations were obtained, and the ratio of urinary β2-MG to TP concentration (µg/mg) was calculated. The Mann-Whitney U test was performed to compare this ratio between these children. The optimal cutoff value of the ratio for considering the presence of glomerular disease was obtained from the receiver operating characteristic (ROC) curve.

RESULTS

We obtained information on 23 children with Dent disease and 14 children with lupus nephritis. The median ratios of urinary β2-MG to TP concentrations in children with Dent disease and lupus nephritis were 84.85 and 1.59, respectively. The ROC curve yielded the optimal cutoff value of this ratio for distinguishing between these diseases, and the cutoff value was found to be 22.3.

CONCLUSION

In children with tubulointerstitial diseases, the urinary β2-MG concentration may be approximately 8.5% of the TP concentration. The possibility of presenting with glomerular disease should be considered in patients with a ratio of urinary β2-MG to TP concentration of < 22.3 (µg/mg).

Kidney disease in adults with Prader-Willi syndrome: international cohort study and systematic literature review

Background

Prader-Willi syndrome (PWS) is a rare, complex, genetic disorder characterized by hyperphagia, hypotonia, delayed psychomotor development, low muscle mass and hypothalamic dysfunction. Adults with PWS often have obesity, hypertension and type 2 diabetes mellitus (DM2), known risk factors for cardiovascular disease (CVD) and chronic kidney disease (CKD). Early symptoms of CVD and CKD may be masked by intellectual disability and inability to express physical complaints. Furthermore, kidney diseases are often asymptomatic. Therefore, renal and cardiovascular disease might be missed in patients with PWS. Microalbuminuria is an early sign of microvascular damage in the kidneys and other vascular beds. Therefore, we screened our adult PWS cohort for the presence of elevated urinary albumin and (micro)albuminuria.

Methods

We retrospectively collected anthropometric measurements, blood pressure, medical history, medication use, urine dipstick and biochemical measurements form electronic patient files. In addition, we performed a systematic literature review on kidney disease in PWS.

Results

We included 162 adults with genetically confirmed PWS (56% male, median age 28 years), of whom 44 (27%) had DM2. None had known CVD. All subjects had normal estimated glomerular filtration rate (eGFR) according to non-PWS reference intervals. Elevated urinary albumin or (micro)albuminuria was present in 28 (18%); 19 out of 75 (25%) had an increased urinary albumin-to-creatinine ratio (UACR) and 10 out of 57 (18%) had an increased urinary protein-to-creatinine ratio. Elevated urinary albumin was present at a young age (median age 26 (IQR 24-32) years) and was associated with an significantly higher BMI and LDL-cholesterol levels and higher prevalence of DM2, hypertension and dyslipidemia than those with normal UACR (p=0.027, p=0.019, p<0.001, p<0.001, p=0.011 and respectively).

Conclusion

Upon screening, one in every five adults with PWS had increased urinary albumin or (micro)albuminuria, early signs of microvascular disease. All had normal eGFR, according to non-PWS reference intervals, and none had a formal diagnosis of CVD. As muscle mass is low in PWS, creatinine levels and eGFR may be spuriously normal. Urinalysis in this patient group can be used as a screening tool for microvascular (kidney) disease. We propose an algorithm for the detection and management of microvascular disease in adults with PWS.

Evaluation of Renal Function Disorder With Urinary Neutrophil Gelatinase-associated Lipocalin Level in Patients With β-Thalassemia Major

BACKGROUND AND AIM

Neutrophil gelatinase-associated lipocalin (NGAL) is found in the granules of human neutrophils, with many diverse functions. Expression of NGAL is induced under various pathophysiological conditions, for example, infection, inflammation, kidney injury, cardiovascular disease, burn injury, and intoxication, and it has an important antiapoptotic and anti-inflammatory role.

PATIENTS AND METHODS

A total of 37 patients with thalassemia disease β-thalassemia major (24 male and 13 female individuals) and 37 healthy controls (17 male and 20 female) were enrolled in this study. All patients were regularly transfused, and all of them were using oral chelator, deferasirox.

RESULTS

NGAL and cystatin C levels were significantly higher in the case group than in the control group.

CONCLUSION

In patients with β-thalassemia major, renal damage can occur owing to regular blood transfusion. Urinary NGAL levels in these patients may be considered as a marker for early renal injury.

Effect and Mechanism of ShiZhiFang on Uric Acid Metabolism in Hyperuricemic Rats

Objective

To explore the effect and mechanism of ShiZhiFang on uric acid metabolism.

Methods

40 rats were divided into normal group, model group, ShiZhiFang group, and benzbromarone group. The hyperuricemic rat model was induced by yeast gavage at 15 g/kg and potassium oxonate intraperitoneal injection at 600 mg/kg for two weeks. During the next two weeks, ShiZhiFang group rats were given ShiZhiFang by gavage, and benzbromarone group rats were given benzbromarone by gavage. The serum uric acid, creatinine, blood urea nitrogen, XOD activity, urinary uric acid, urinary β₂-MG, and histopathological changes were observed in the rats of each group after treatment.

Results

The hyperuricemic model was established successfully and did not show the increase of serum creatinine and blood urea nitrogen. Compared with the model group, the serum uric acid, serum XOD activity, and urinary β₂-MG were significantly decreased (p < 0.05), and 24 h urinary uric acid excretion was significantly decreased (p < 0.01) in ShiZhiFang group, whereas the two treatment groups were of no statistical significant in above indicators (p > 0.05); renal histopathology showed that the lesions in two treatment groups were reduced compared to the model groups. The gene and protein expression of uric acid anion transporters rOAT1 and rOAT3 in the kidney was significantly higher than that in model group (p < 0.01).

Conclusion

The model is suitable for the study of primary hyperuricemia. The mechanisms of ShiZhiFang on uric acid metabolism in hyperuricemic rats may be involved in reducing the activity of serum XOD and promoting the transcription and expression of rOAT1 and rOAT3 in the kidney.

Beta-2-microglobulin (B2M) expression in the urinary sediment correlates with clinical markers of kidney disease in patients with type 1 diabetes

PURPOSE

After observing variation in the expression of the housekeeping gene B2M in cells of the urinary sediment during a study of candidate genes potentially involved in diabetic kidney disease (DKD), we hypothesized that B2M mRNA expression in the urinary sediment could reflect the presence of DKD.

METHODS

qPCR was used to quantify B2M mRNA expression in cells of the urinary sediment of 51 type 1 diabetes (T1D) patients (61% women, 33.5 [27.0-39.7] years old, with diabetes duration of 21.0 [15.0-28.0] years and HbA1c of 8.2% [7.3-8.9]; median [interquartile interval]) sorted according to the diabetic nephropathy (DN) stages; 8 focal segmental glomerulosclerosis (FSGS) patients and 10 healthy controls. B2M mRNA expression was also evaluated in human embryonic kidney epithelium-like (HEK-293) cells exposed to 25mM glucose and to albumin in order to mimic, respectively, a diabetic and a proteinuric milieu.

RESULTS

No differences were found in B2M mRNA expression among healthy controls, FSGS and T1D patients. Nonetheless B2M mRNA expression was higher in the group composed by T1D patients with incipient or overt DN combined with FSGS patients versus T1D patients without DN combined with healthy controls (P=0.0007). B2M mRNA expression was higher in T1D patients with incipient or overt DN versus without DN (P=0.03). B2M mRNA expression positively correlated with albuminuria in the overall T1D population (r=0.43; P=0.01) and negatively correlated with estimated glomerular filtration rate in male T1D patients (r=- 0.57; P=0.01). Increased B2M expression was observed in HEK-293 cells exposed to 25mM glucose and to albumin.

CONCLUSIONS

Β2M mRNA expression in cells of the urinary sediment is higher in T1D patients with DKD and in patients with FSGS in comparison to healthy subjects, maybe reflecting a tubulointerstitial injury promoted by albumin. Given the proinflammatory nature of B2M, we suggest that this protein contributes to diabetic (and possibly, to non-diabetic) tubulopathy.

The ratios of urinary β2-microglobulin and NAG to creatinine vary with age in children

BACKGROUND

Although urinary biochemical markers can be assessed by their ratio to urinary creatinine (U-Cr) concentration, reference values in adults may not be applicable to children because the amount of Cr excreted varies by body size. We therefore measured the relationship between age and the ratios of urinary β-2-microglobulin (U-β2MG), N-acetyl-β-d-glucosaminidase (U-NAG), calcium (U-Ca) and protein (U-Pr) concentration to those of U-Cr in children.

METHODS

Fifty-seven patients aged >1 year with benign familial hematuria (median age, 6.3 years) were divided into three age groups: 1-4, 5-9, and ≥10 years. Urinary biomarkers were assayed using actual values; ratios to actual U-Cr values; and our standardized metric, namely 100-fold the ratio of serum Cr to U-Cr concentration; and the relationship of each of these to age was determined.

RESULTS

The ratios of U-β2MG, U-NAG and U-Ca to Cr varied significantly by age, being higher in younger than in older children, but the actual and standardized values of each did not vary by age, nor did any measurement of U-Pr.

CONCLUSIONS

The ratios of urinary markers of tubular function, including U-β2MG, U-NAG and U-Ca, to Cr vary by age, being higher in younger children. In contrast, the ratios of urinary markers of glomerular filtration (such as U-Pr)to Cr do not vary by age, making them suitable for corrections relative to Cr.

Urine retinol-binding protein 4: a functional biomarker of the proximal renal tubule

Measurement of retinol-binding protein 4 in urine (uRBP4) is arguably the most sensitive biomarker for loss of function of the human proximal renal tubule. Megalin- and cubilin-receptor-mediated endocytosis normally absorbs > 99% of the approximately 1.5 g/24 h of protein filtered by the renal glomerulus. When this fails there is "tubular proteinuria," comprising uRBP4, albumin, and many other proteins and peptides. This tubular proteinuria is a consistent feature of the renal Fanconi syndrome (FS) and measurement of uRBP4 appears to be an excellent screening test for FS. FS occurs in rare inherited renal diseases including cystinosis, Dent disease, Lowe syndrome, and autosomal dominant FS. Acquired FS occurs in paraproteinemias, tubulointerstitial renal disease, oncogenic osteomalacia, Chinese herbs nephropathy, and Balkan endemic nephropathy. Though poorly understood, FS may be associated with HIV disease and antiretroviral treatment; cadmium poisoning may cause FS. In addition to FS, uRBP4 measurement has a different role: the early detection of acute kidney injury. Urine RBP4 comprises several isoforms, including intact plasma RBP4, MW 21.07 kDa, and C-terminal truncated forms, des-L- and des-LL-RBP4, also probably plasma derived. In FS, uRBP4 levels are about 104-fold above the upper limit of normal and small increments are frequently seen in carriers of some inherited forms of FS and in acquired disease. The very high levels in disease, frequent assay nonlinearity, lack of defined calibrants, and multiple uRBP4 isoforms make accurate assay challenging; top-down mass spectrometry has brought advances. Assays for uRBP4 with defined molecular targets allowing good interlaboratory comparisons are needed.

Urinary retinol binding protein is a marker of the extent of interstitial kidney fibrosis

Currently, a non-invasive method to estimate the degree of interstitial fibrosis (IF) in chronic kidney disease is not available in routine. The aim of our study was to evaluate the diagnostic performance of the measurement of urinary low molecular weight (LMW) protein concentrations as a method to determine the extent of IF. The urines specimen from 162 consecutive patients who underwent renal biopsy were used in the analysis. Numerical quantification software based on the colorimetric analysis of fibrous areas was used to assess the percentage IF. Total proteinuria, albuminuria, and the urinary levels of retinol binding protein (RBP), alpha1-microglobulin (α1MG), beta 2-microglobulin (β2MG), transferrin, and IgG immunoglobulins were measured. There was a significant correlation between the degree of IF and the RBP/creatinine (creat) ratio (R2: 0.11, p<0.0001). IF was associated to a lesser extent with urinary β2MG and α1MG; however, there was no association with total proteinuria or high molecular weight (HMW) proteinuria. The correlation between IF and RBP/creat remained significant after adjustment to the estimated glomerular filtration rate, age, body mass index, α1MG, and β2MG. The specificity of the test for diagnosing a fibrosis score of >25% of the parenchyma was 95% when using a threshold of 20 mg/g creat. In conclusion, RBP appears to be a quantitative and non-invasive marker for the independent prediction of the extent of kidney IF. Because methods for the measurement of urinary RBP are available in most clinical chemistry departments, RBP measurement is appealing for implementation in the routine care of patients with chronic kidney disease.

Follow-up results of children with melamine induced urolithiasis: a prospective observational cohort study

BACKGROUND

Melamine-contaminated milk powder was the cause of the 2008 outbreak of urolithiasis in young children and infants in China, but the prognosis of these children remains unknown. We hypothesized that urolithiasis induced by melamine-contaminated milk powder may be associated with secondary renal injury.

METHODS

A total of 8335 children (≤6 years old) with a history of consuming melamine-contaminated milk powder were screened. Urine analysis and urinary system ultrasonography were performed. For children with urolithiasis, the basic information and the results of examination were recorded, and effective therapy was given. They were followed up for 6 months after the original diagnosis, and urinary microprotein profiles were measured.

RESULTS

Of the 8335 children, 105 (1.26%) were diagnosed with melamine-contaminated milk powder-associated urolithiasis. The size of the stone was correlated with the duration of exposure to melamine. Six months later, 69.8% (67) of the children with urolithiasis passed stones (follow-up rate: 91.4%). Of the 67 children, 28 passed stones within 2 months. The higher possibility of passing a stone was correlated with the smaller diameter of the stone (P<0.001). The detection rate of abnormal urinary microprotein excretion (microalbumin, immunoglobulin G, and N-acetyl-β-D-glucosidase) was 52.4% in children with persistent stones and 38.2% in those who passed their stones. The detection rate was lower in children who passed stones within 2 months (31.8%) than in those who passed stones in 2 to 6 months (50.0%). The levels of microalbumin/creatinine and immunoglobulin G/creatinine were significantly higher in children with persistent stones than in those who passed their stones.

CONCLUSIONS

Early passage of a stone may reduce the renal injury induced by melamine-contaminated milk powder-associated urolithiasis.

Biomarkers of kidney integrity in children and adolescents with dental amalgam mercury exposure: findings from the Casa Pia children's amalgam trial

Mercury is toxic to the kidney, and dental amalgam is a source of mercury exposure. Few studies have evaluated the effects of dental amalgam on kidney function in a longitudinal context in children. Here, we evaluated urinary concentrations of glutathione S-transferases (GSTs) alpha and pi as biomarkers of renal proximal and distal tubular integrity, respectively, and albumin as a biomarker of glomerular integrity in children and adolescents 8-18 years of age over a 7-year course of dental amalgam treatment. Five hundred seven children, 8-12 years of age at baseline, participated in a clinical trial to evaluate the neurobehavioral and renal effects of dental amalgam in children. Subjects were randomized to either dental amalgam or resin composite treatments. Urinary GSTs alpha and pi, albumin, and creatinine concentrations were measured at baseline and annually in all subjects. Results were evaluated using linear regression analysis. GST-alpha concentrations were similar between treatment groups and in each sex and race (white vs. non-white) group in each follow-up year. GST-pi levels tended upward over the course of follow-up by four- to six-fold. This increase was seen in all groups irrespective of the treatment, race, or gender. Females had GST-pi levels approximately twice those of males at all ages. Albumin concentrations were constant throughout the follow-up period and did not differ by treatment, although females had 39% higher albumin levels than males. Additionally, we found no significant effects of amalgam treatment on the proportion of children with microalbuminuria (>30 mg/g creatinine). These findings are relevant within the context of children's health risk assessment as relates to the safety of mercury exposure from dental amalgam on kidney function. These data also provide normative values for sensitive indices of renal functional integrity that may serve in the evaluation of children and adolescents with renal disorders.

Grand rounds: nephrotoxicity in a young child exposed to uranium from contaminated well water

CONTEXT

Private wells that tap groundwater are largely exempt from federal drinking-water regulations, and in most states well water is not subject to much of the mandatory testing required of public water systems. Families that rely on private wells are thus at risk of exposure to a variety of unmeasured contaminants.

CASE PRESENTATION

A family of seven--two adults and five children--residing in rural northwestern Connecticut discovered elevated concentrations of uranium in their drinking water, with levels measured at 866 and 1,160 microg/L, values well above the U.S. Environmental Protection Agency maximum contaminant level for uranium in public water supplies of 30 microg/L. The uranium was of natural origin, and the source of exposure was found to be a 500-foot well that tapped groundwater from the Brookfield Gneiss, a geologic formation known to contain uranium. Other nearby wells also had elevated uranium, arsenic, and radon levels, though concentrations varied widely. At least one 24-hr urine uranium level was elevated (> 1 microg/24 hr) in six of seven family members (range, 1.1-2.5 microg/24 hr). To assess possible renal injury, we measured urinary beta-2-microglobulin. Levels were elevated (> 120 microg/L) in five of seven family members, but after correction for creatine excretion, the beta-2-microglobulin excretion rate remained elevated (> 40 microg/mmol creatinine) only in the youngest child, a 3-year-old with a corrected level of 90 microg/mmol creatinine. Three months after cessation of well water consumption, this child's corrected beta-2-microglobulin level had fallen to 52 microg/mmol creatinine.

SIGNIFICANCE

This case underscores the hazards of consuming groundwater from private wells. It documents the potential for significant residential exposure to naturally occurring uranium in well water. It highlights the special sensitivity of young children to residential environmental exposures, a reflection of the large amount of time they spend in their homes, the developmental immaturity of their kidneys and other organ systems, and the large volume of water they consume relative to body mass.

Evaluation of renal function in Turkish children receiving BFM-95 therapy for acute lymphoblastic leukemia

This study examined renal function in 42 children with acute lymphoblastic leukemia (ALL) treated according to BFM-95 protocol. Fifteen (group 1) were investigated longitudinally at 3 time points: before (T1), 4 weeks after (T2), and 2-6 months after (T3) consolidation therapy with high-dose methotrexate (HDMTX). The frequency of abnormalities in glomerular and tubular tests were nil at T1 and ranged from 13 to 40% at T2 and 7 to 33% at T3 in group 1. Twenty percent of the patients (n = 10) in group 2, who were examined at a single time point 7-36 months after consolidation, had glomerular and tubular abnormalities. There was only mild tubular abnormality in 5.8% of patients (n = 17) in group 3, who were examined at a single time point a mean of 56.1 +/- 12.5 months after completion chemotherapy. These data show that consolidation therapy with HDMTX is frequently associated with acute renal toxicity in children with ALL but does not leave clinically significant late sequelae.

Renal tubular dysfunction in alpha-thalassemia

Shortened red cell life span and excess iron cause functional and physiological abnormalities in various organ systems in thalassemia patients. In an earlier study, we showed that beta-thalassemia patients have a high prevalence of renal tubular abnormalities. The severity correlated with the degree of anemia, being least severe in patients on hypertransfusion and iron chelation therapy, suggesting that the damage might be caused by the anemia and increased oxidation induced by excess iron deposits. This study was designed to define the renal abnormalities associated with alpha-thalassemia and to correlate the renal findings with clinical parameters. Thirty-four pediatric patients (mean age 8.2+/-2.8 years) with Hb H disease or Hb H/Hb CS were studied. Ten patients (group 1) were splenectomized, with a mean duration post splenectomy of 3.5+/-1.4 years; 24 patients (group 2) had intact spleens. The results were compared with 15 normal children. Significantly higher levels of urine N-acetyl-beta- d-glycosaminidase, malondialdehyde (MDA), and beta(2)-microglobulin were found in both groups compared with normal children. An elevated urine protein/creatinine ratio was recorded in 60% of group 1 and 29% of group 2. Two patients (5.9%), 1 in each group, had generalized aminoaciduria. We found proximal tubular abnormalities in alpha-thalassemia patients. Increased oxidative stress, possibly iron induced, may play an important role, since urine MDA levels were significantly increased in both groups of patients.

Evaluating the child with proteinuria

Proteinuria is a common laboratory finding in children. It might represent a benign condition or herald the presence of a serious underlying renal disease or systemic disorder. Investigation to confirm a diagnosis or seek reassurance is important. This paper discusses the various causes of proteinuria, and those aspects of the history, physical examination, and the laboratory tests that will help determine the cause or reassure that a serious problem is not currently present.

Age-related reference limits for urine levels of albumin, orosomucoid, immunoglobulin G and protein HC in children

Urine levels of albumin, orosomucoid (alias alpha1-acid glycoprotein), immunoglobulin G (IgG) and protein HC (alias alpha1-microglobulin) were determined in 247 healthy Swedish infants and children aged 1 day to 15 years. Urine samples were collected and stored in conditions known to guarantee stable protein levels. The protein levels were measured both as mass concentrations (mg l(-1)) and as protein-creatinine ratios (mg mmol(-1)). In an effort to arrive at practically useful upper reference limits, the variability with age for both units was analysed. The variability with age for the levels of three of the four proteins was considerably lower when the levels were expressed as creatinine ratios rather than as mass concentrations. The results allowed suggestion of the following upper reference limits, expressed as mg mmol(-1) creatinine, for use in clinical practise: IgG, 1 month to 15 years: 1.0; protein HC, 1 month to 15 years: 0.8; orosomucoid, 1 month to 15 years: 0.5; albumin, 1 month to 1 year: 3.8; 1-5 years: 3.3; 6-10 years: 2.7; 11-15 years: 2.1. In the immediate neonatal period the urine levels of all proteins were high and very variable.

Renal tract abnormalities detected in Australian preschool children

OBJECTIVE

To quantify the incidence of abnormalities in urinalysis and blood pressure from preschool children and their predictive value in detecting renal disease within an Australian community.

METHODOLOGY

Urine samples, blood pressure and height measurements and parental reports of significant medical problems were collected from a total of 9355 South Australian preschool children. Seven hundred and forty-three children with abnormal results were investigated in a nephrology outpatient clinic. A control group of 357 children with no detectable abnormality were also recalled, examined and, where appropriate, investigated.

RESULTS

Nine thousand, three hundred and fifty-five children were tested. Of these, 0.81% were shown to have a clinically significant renal tract abnormality. The findings included children with urinary tract infections, vesico-ureteric reflux, glomerular disease, renal calculi, essential hypertension and a renal neoplasm. While dipstick-based methods were the most specific indicators of renal tract abnormalities, measurement of blood pressure and urinary beta2-microglobulin were also important in detecting abnormalities. Screening for glycosuria did not result in the detection of significant undiagnosed abnormalities. In the control group with no abnormality detected at testing, there was one case each of aortic coarctation, polycystic kidney disease and vesico-ureteric reflux diagnosed.

CONCLUSION

Undiagnosed renal tract abnormalities are present in many Australian preschool children. Most are detectable by a thorough history, examination and urinalysis.

X-linked recessive nephrolithiasis: presentation and diagnosis in children

We report a new X-linked recessive nephrolithiasis kindred. X-linked recessive nephrolithiasis is a recently described disease characterized by recurrent nephrolithiasis, nephrocalcinosis, and progressive renal failure, associated with mutations in a renal chloride channel gene, chloride channel number 5. Screening individuals at risk with renal ultrasonography and measurement of urinary excretion of low molecular weight proteins and calcium will exclude boys without X-linked recessive nephrolithiasis kindred and identify boys likely to have the disease.

Measurement of albumin and low molecular weight proteins in the urine of newborn infants using a cotton wool ball collection method

OBJECTIVE

The aim of this study was to investigate the inter-relationship between urinary excretion of alpha-1-microglobulin (A1M), retinol-binding protein (RBP) and albumin in term and premature neonates, with urine collected into cotton wool balls and extracted by a novel method.

SUBJECTS AND METHODS

Sixty-four infants were studied on the first day of life; 26 had been born at term (37-42 weeks gestation) and 38 prematurely (24-28 weeks n = 16, 29-36 weeks n = 22). Urine collected into cotton wool balls was analysed following a new detergent extraction method, which resulted in a recovery rate of 94-107% for albumin. A1M, RBP and creatinine.

RESULTS

Urinary protein excretion, expressed as a ratio to urinary creatinine, decreased significantly with increasing gestational age (24-28 weeks, 29-36 weeks, 37-42 weeks: albumin:creatinine ratio mg/mmol mean 96.9, 31.7, 19.3; A1M:creatinine ratio mg/mmol mean 99.3, 37.0, 7.8; RBP:creatinine ratio mg/mmol mean 16.2, 3.8, and < 0.01, below the limit of detection, respectively). When results were corrected for birthweight, this gestation-associated effect was still present for A1M and RBP, but not for albumin. In premature infants there was a significant positive correlation between A1M:creatinine ratio and RBP:creatinine ratio (r = 0.85), and also between albumin and both A1M and RBP (r = 0.82 and 0.77).

CONCLUSION

Increased excretion of A1M, RBP and albumin at earlier gestational ages is probably due to proximal tubular immaturity, although tubular damage and also glomerular dysfunction cannot be excluded as possible explanations.

Assessment of retinol-binding protein excretion in normal children

Retinol-binding protein (RBP) is a low molecular weight protein freely filtered at the glomerulus. The fractional tubular reabsorption of RBP is 99.97% and increased excretion is therefore a sensitive marker of tubular dysfunction. We obtained early-morning urine specimens from 151 well children, from newborn to 16 years of age. RBP was measured using an enzyme-linked immunosorbent assay, albumin by a radioimmunoassay and creatinine by a modified Jaffé reaction. Protein excretion was assessed by calculating the protein:creatinine ratio for early-morning urine samples. We found a fall in both RBP and albumin excretion with increasing age, particularly in the 1st year of life, with a much wider variation in values from the infants studied. The mean excretion of RBP for children aged 0-6 months [51.4 (0.6-4,719) micrograms/mmol] was significantly higher (P < 0.001) than the mean for children aged 6 months to 16-years [15.0 (3.8-60) micrograms/mmol]. It has been shown that measurement of tubular proteinuria using the RBP: creatinine ratio is useful in the assessment of children with renal disease and we propose a value two standard deviations above the geometric mean for the age of the patient as an upper limit of normal.