Rapid radioimmunoassay for diagnosing acute tubular necrosis

Diagnostic Performance of Fractional Excretion of Sodium for the Differential Diagnosis of Acute Kidney Injury: A Systematic Review and Meta-Analysis

BACKGROUND AND OBJECTIVES

AKI is classified as prerenal, intrinsic, and postrenal. Prerenal AKI and intrinsic AKI represent the most common causes for AKI in hospitalized patients. This study aimed to examine the accuracy of the fractional excretion of sodium for distinguishing intrinsic from prerenal AKI.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We searched MEDLINE, Embase, Cochrane Central Register of Controlled Trials, the Cochrane Library, and Scopus for all available studies that met the criteria until December 31, 2021. We included studies that evaluated fractional excretion of sodium in differentiating AKI etiologies in adults, whereas studies that did not have sufficient data to extract a 2×2 table were excluded. We assessed the methodologic quality using the Quality Assessment of Diagnostic Accuracy Studies-2 tool and extracted the diagnostic accuracy data for all included studies. We conducted a meta-analysis using the bivariate random effects model. We performed subgroup analysis to investigate sources of heterogeneity and the effect of the relevant confounders on fractional excretion of sodium accuracy.

RESULTS

We included 19 studies with 1287 patients. In a subset of 15 studies (872 patients) that used a threshold of 1%, the pooled sensitivity and specificity for differentiating intrinsic from prerenal AKI were 90% (95% confidence interval, 81% to 95%) and 82% (95% confidence interval, 70% to 90%), respectively. In a subgroup of six studies (511 patients) that included CKD or patients on diuretics, the pooled sensitivity and specificity were 83% (95% confidence interval, 64% to 93%) and 66% (95% confidence interval, 51% to 78%), respectively. In five studies with 238 patients on diuretics, the pooled sensitivity and specificity were 80% (95% confidence interval, 69% to 87%) and 54% (95% confidence interval, 31% to 75%), respectively. In eight studies with 264 oliguric patients with no history of CKD or diuretic therapy, the pooled sensitivity and specificity were 95% (95% confidence interval, 82% to 99%) and 91% (95% confidence interval, 83% to 95%), respectively.

CONCLUSIONS

Fractional excretion of sodium has a limited role for AKI differentiation in patients with a history of CKD or those on diuretic therapy. It is most valuable when oliguria is present.

Urinary biomarkers in septic acute kidney injury

OBJECTIVE

To appraise the literature on the value of urinary biomarkers in septic acute kidney injury (AKI).

DESIGN

Systematic review.

SETTING

Academic medical centre.

PATIENTS AND PARTICIPANTS

Human studies of urinary biomarkers.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Fourteen articles fulfilled inclusion criteria. Most studies were small, single-centre, and included mixed medical/surgical adult populations. Few focused solely on septic AKI and all had notable limitations. Retrieved articles included data on low-molecular-weight proteins (beta2-microglobulin, alpha1-microglobulin, adenosine deaminase binding protein, retinol binding protein, cystatin C, renal tubular epithelial antigen-1), enzymes (N-acetyl-beta-glucosaminidase, alanine-aminopeptidase, alkaline phosphatase; lactate dehydrogenase, alpha/pi-glutathione-S-transferase, gamma-glutamyl transpeptidase), cytokines [platelet activating factor (PAF), interleukin-18 (IL-18)] and other biomarkers [kidney injury molecule-1, Na/H exchanger isoform-3 (NHE3)]. Increased PAF, IL-18, and NHE3 were detected early in septic AKI and preceded overt kidney failure. Several additional biomarkers were evident early in AKI; however, their diagnostic value in sepsis remains unknown. In one study, IL-18 excretion was higher in septic than in non-septic AKI. IL-18 also predicted deterioration in kidney function, with increased values preceding clinically significant kidney failure by 24-48 h. Detection of cystatin C, alpha1-microglobulin, and IL-18 predicted need for renal replacement therapy (RRT).

CONCLUSIONS

Few clinical studies of urinary biomarkers in AKI have included septic patients. However, there is promising evidence that selected biomarkers may aid in the early detection of AKI in sepsis and may have value for predicting subsequent deterioration in kidney function. Additional prospective studies are needed to accurately describe their diagnostic and prognostic value in septic AKI.

Urinary Biochemistry and Microscopy in Septic Acute Renal Failure: A Systematic Review

BACKGROUND

Biochemistry and microscopy of urine are widely published diagnostic activities in patients with acute renal failure (ARF). However, their scientific basis in patients with septic ARF has not been assessed systematically.

METHODS

We performed a systematic review of MEDLINE, EMBASE, CINHAL, and PubMed databases and bibliographies of retrieved articles for all studies describing urinary biochemistry, indices, and microscopy in patients with septic ARF.

RESULTS

We identified 27 articles (1,432 patients). Because of substantial heterogeneity, no formal quantitative analysis could be performed. Urinary biochemistry or derived indices were reported in 24 articles (89%), and microscopy, in 7 articles (26%). The majority were small single-center reports and had serious limitations. For example, only 52% of patients were septic, only 54% of patients had ARF, many studies failed to include a control group, time from diagnosis of sepsis or ARF to measure of urinary tests was variable, and there were numerous potential confounders. Urinary sodium, fractional excretion of sodium, urinary-plasma creatinine ratio, urinary osmolality, urinary-plasma osmolality ratio, and serum urea-creatinine ratio showed variable and inconsistent results. Low-molecular-weight proteinuria was described in only 22% of articles. A few reports of urinary microscopy described muddy brown/epithelial cell casts and renal tubular cells in patients with septic ARF, whereas others described normal urinary sediment.

CONCLUSION

The scientific basis for the use of urinary biochemistry, indices, and microscopy in patients with septic ARF is weak. More research is required to describe their accuracy, pattern, and time course in patients with septic ARF.

Acute renal failure

ARF is a common clinical problem associated with significant morbidity and mortality. Therefore, rapid and accurate diagnosis is imperative. Initial diagnostic strategies should be directed toward distinguishing among prerenal, renal, and postrenal causes of ARF. This can be done by the use of urinary diagnostic indices in concert with radiologic investigation, renal biopsy, and hemodynamic monitoring when appropriate. Treatment of established ARF should be directed toward maintaining fluid and electrolyte homeostasis and controlling complications resulting from retained nitrogenous waste products. This may necessitate the aggressive use of dialysis. As yet, there is no proven means to prevent ARF or alter the course of ARF once it is established.

Nephrotoxicity from angiographic contrast material. A prospective study

Three hundred and seventy-eight hospitalized patients undergoing nonrenal angiography were evaluated for subsequent changes in renal function. Acute renal failure was defined as a rise in the serum creatinine level of 1.0 mg/dl or more. Several factors that appeared to play no significant role in causing acute renal failure included: the volume of contrast material injected, the anatomic site of injection and the presence of a prior history of cardiovascular disease or diabetes mellitus. The single risk factor identified was the presence of preexistent azotemia (blood urea nitrogen of 30 mg/dl and serum creatinine of 1.5 mg/dl). Whereas nonazotemic patients had a 2 percent incidence of definite acute renal failure, patients with chronic azotemia (mean blood urea nitrogen/creatinine = 47/2.3 mg/dl) had a 33 percent incidence. Three patients required short-term dialysis, and two required potassium-exchange resin therapy. No patient required permanent dialysis, and no patient died of acute renal failure. The persistence of a positive nephrogram 24 hours after angiography was a sensitive detector of a rise in the serum creatinine level although more expensive than the creatinine determination. While urine sediment analysis confirmed the diagnosis in many cases, it was relatively insensitive. Monitoring of urine volume proved to be of little value. We recommend a screening serum creatinine determination 24 to 48 hours after infusion of angiographic contrast material in azotemic patients.

Quantitating the severity of proximal tubular brush border injury by a simple direct binding radioimmunoassay

The purpose of this investigation was: (1) to establish a simplified radioimmunoassay (RIA) for quantitating renal tubular epithelial antigens (RTE) in urine; (2) to ascertain whether urine RTE concentrations as measured by this technique correlate with the severity of acute nephrotoxic and ischemic injury; and (3) to ascertain whether increased urinary RTE is a specific marker of renal tubular injury. A direct binding RTE RIA was established using 125I labelled anti-RTE antibody and 10% polyethylene glycol to separate bound from unbound anti-RTE 125I. This RIA is simpler than previously described RTE assay methods since: (1) double antibody separation techniques are eliminated; (2) RTE antigen purification from crude proximal tubular fragments is no longer necessary; and (3) immunoglobulin G(IgG) rather than more radiosensitive RTE is used as the radioligand. To test the utility of this assay as a marker of acute tubular injury anesthetized rats were subjected to graded nephrotoxic (HgCl2: 0--20 mg/kg) or bilateral renal ischemic (0--32 min) insults. Glomerular filtration rates (GFR) (clearance iothalamate 125I) and RTE concentrations were measured sequentially. Post-renal injury, RTE concentrations rose above control values and the degree of elevation strongly correlated with the severity of GFR depression (r = 0.72--0.81; p less than 0.02--0.05). The source of this increased urinary RTE was the proximal tubule since brush border loss was demonstrated histologically and because no RTE could be detected in serum. Rats whose GFRs were acutely depressed by inducing either volume depletion or acute experimental glomerulonephritis (nephrotoxic serum nephritis) all had normal urine RTE concentrations. These results suggest that RTE quantitation by this technique may provide a specific and early quantitative index of the severity of acute nephrotoxic and ischemic renal injury.

Susceptibility of the proximal tubular brush border to acute obstructive injury

The purpose of this investigation was to determine the susceptibility of proximal renal tubular epithelial cells to the effects of acute urinary tract obstruction. Brush border derived-renal tubular epithelial antigens (RTE) were quantitated in urine by radioimmunoassay both before and after the induction of bilateral ureteral obstruction of 15-150 minutes duration in anesthetized diuretic rats. Glomerular filtration rates (GFR) before and after obstruction were determined by calculating renal clearances of Na iothalamate 125I. Light microscopic examination of renal tissue was performed. Post obstructive renal function was well preserved in all rats with GFRs ranging from 76-100 per cent of control values. However, in the immediate post-obstructive period RTE excretion increased dramatically in all rats. Up to 10 fold increases in urine antigen concentrations were observed under conditions of isosthenuria and stable urine flow rates. The degree of elevation correlated with the length of obstruction and with the severity of histologic changes in the proximal tubules (progressive proximal tubular dilatation and brush border loss). It is concluded that proximal tubular cells are highly susceptible to obstructive insults as brief as 15 minutes in duration. The mechanism of this injury and its effect on post-obstructive renal function remain to be clarified.

A search for nephrotoxic factors within the uremic milieu

The purpose of this study was to ascertain whether selected components of the uremic milieu adversely affected glomerular filtration rate (GFR), the glomerular protein filtration barrier, or the integrity of the proximal renal tubular brush border membrane. To achieve these goals, GFR and the excretion rates of albumin and of brush border derived-renal tubular epithelial antigens (RTE) were measured in normal rats and in rats with experimental nephropathies before and after the intravenous infusion of concentrated urine. This experimental protocol uniformly produced severe biochemical manifestations of uremia (for example 10-50-fold increases in BUN and creatinine, hyperphosphatemia, hyperkalemia, metabolic acidosis). However, despite these perturbations, GFR, albuminuria, and RTE excretion remained constant. To assess the influence of uremic hormonal derangements on renal function, GFR, albuminuria, and RTE excretion were measured in normal rats before and after inducing acute serum elevations of seven hormones whose concentrations are known to be increased in uremia (parathyroid hormone, growth hormone, insulin, glucagon, gastrin, prolactin, gastric inhibitory peptide). Again, GFR, albuminuria, and RTE excretion were not adversely affected. These results suggest that glomerular capillary function and proximal tubular brush border membranes are acutely resistant to many of the solute and hormonal derangements which are characteristic of uremia.