Effects of storage at -20 degrees C on ischaemia-modified albumin results.

BACKGROUND: Ischaemia-modified albumin (IMA) is being studied as a new marker for reversible ischaemia in patients presenting with possible cardiac chest pain. The conditions under which samples are stored prior to analysis may be critical in influencing the analytical result and hence the cut-off used in any particular study. METHODS: Sixty-eight samples taken during a study assessing the performance of IMA for risk stratification in patients presenting with possible cardiac chest pain were analysed both within 2.5 h of collection and after periods of storage at -20 degrees C. RESULTS: Samples stored at -20 degrees C yielded IMA values on average 3 units higher than those analysed within 2.5 h (mean 90.5 vs. 87.5; P < 0.00001). A Bland-Altman plot showed that the difference was not concentration dependent. CONCLUSIONS: These results indicate that decision cut-offs will be influenced by conditions of sample storage prior to IMA analysis, and that these should be stated in detail for each study.

The PRIMA study: presentation ischaemia-modified albumin in the emergency department.

OBJECTIVES: To investigate the diagnostic accuracy of presentation ischaemia-modified albumin (IMA), in addition to cardiac troponin I (TnI), as a strategy to rapidly ascribe low risk to patients with chest pain attending an emergency department, and to determine whether IMA has the potential to reduce transit time in emergency departments. METHODS: A prospective observational study was carried out in two emergency departments (belonging to the John Radcliffe Hospital, Oxford, UK; and the Frenchay Hospital, Bristol, UK) of similar size. Consecutive adult patients presenting with features of possible ischaemic cardiac chest pain and a normal electrocardiogram were eligible. The index test (measurement of IMA and TnI at presentation) and reference standard (delayed TnI measurement, taken at least 8 h after pain onset) were applied to all recruited patients. All clinicians were blinded to the results of the index test. Assays were carried out in a single laboratory using standard techniques. RESULTS: 399 patients were recruited; 277 patients had a result for both the index test and reference standard. The sensitivity was 97.6% (95% confidence interval (CI) 87.4 to 99.9), negative predictive value 97% (95% CI 84.2 to 99.9) and specificity 13.6% (95% CI 9.5 to 18.7). Sensitivity analysis showed similar findings in three alternative scenarios. Receiver operating characteristic analysis indicated that a different "cut-off" value for IMA would not improve the properties of the test. The median potential time saved (n = 268) was 6 h and 10 min. CONCLUSION: The diagnostic accuracy of presentation IMA in this study does not support its use as an effective risk stratification tool for patients with chest pain in the emergency department. The sensitivity is insufficiently high, with a small number of false negatives undermining the safety of the test. Frequent false positives produce a low specificity that limits the practical value of the test.

Increased urinary albumin and retinol-binding protein in type I diabetes. A study of identical twins.

OBJECTIVE: Indexes of early renal glomerular and tubular dysfunction have been demonstrated in type I diabetes, but it remains uncertain whether such changes are genetically determined or are secondary to the disease process. We therefore undertook to study whether early markers of renal dysfunction are a consequence of type I diabetes or inherited. RESEARCH DESIGN AND METHODS: We estimated both urinary albumin excretion (UAE) and urinary retinol-binding protein (RBP) in 51 identical twin pairs discordant for type I diabetes and in 51 matched control subjects. RESULTS: UAE and RBP were significantly higher in the diabetic twins than in their nondiabetic co-twins (P < 0.0001 and P < 0.0002, respectively). Seven diabetic twins had elevated UAE, but none of the nondiabetic co-twins did. In a subgroup of 44 twins with normal UAE (albumin excretion rate < 20 micrograms/min), diabetic twins had both a higher albumin excretion function (median [range]; 0.64 [0.18-2.74] mg/mmol creatinine) than their nondiabetic co-twins (0.48 [0.24-1.40], P < 0.01) and higher levels of RBP excretion (10.4 [4.0-167.0] micrograms/mmol creatinine) than their nondiabetic co-twins (7.5 [0.97-23.0], P < 0.05). Values between twins of a pair were significantly correlated for RBP (r = 0.36, P < 0.05) but not for UAE (r = 0.13). CONCLUSIONS: These results suggest that in type I diabetes, an index of renal tubular function (RBP), but not glomerular function (UAE), is influenced by shared genetic and nongenetic factors. Type I diabetes can affect renal tubular function even when glomerular function is normal. We conclude that neither the increased UAE nor urinary RBP found in type I diabetes is inherited independently of the diabetes process.

Urinary excretion of albumin and retinol-binding protein during normal pregnancy.

In a cross sectional study of 88 pregnant women urinary excretion of albumin, when expressed as a ratio to creatinine concentration, was not significantly different from that in a non-pregnant control group of similar age (p greater than 0.05) and did not change significantly during pregnancy. Only when albumin excretion was expressed as a fractional clearance was the urinary excretion significantly increased in the third trimester compared with the first trimester (p less than 0.05), although it was still not significantly different from that in the non-pregnant control group. Excretion of retinol-binding protein was significantly increased during all three trimesters of pregnancy (p less than 0.01 in each case) and more so in the second and third trimesters than in the first. It is concluded that the increased total protein excretion that has been described during pregnancy is not explained by an increased excretion of albumin which remains essentially normal. In contrast, the tubular absorption of proteins is decreased.

Urine concentrations of apolipoprotein AI in renal tubular proteinuria.

To test the hypothesis that a significant proportion of apolipoprotein AI (apoAI) metabolism occurs through glomerular filtration of free apoAI in serum and subsequent renal tubular metabolism, we have examined the urine concentration of apoAI in three situations in which proximal tubular reabsorption of another protein metabolized in this manner and retinol-binding protein (RBP) was impaired. Following infusion of a cross-linked gelatin polymer (Haemaccel) in four normal subjects, urine RBP excretion (normally about 100 micrograms/L), was between 14 and 46 mg/L, while urine apoAI excretion was less than 0.5 mg/L. On the third day following cardiac surgery involving Haemaccel infusion, urine RBP was between 27 and 159 mg/L while urine apoAI excretion was again less than 0.5 mg/L. In 16 samples from eight patients recently transplanted with allograft kidneys, urine RBP was between 9 and 70 mg/L, whereas in only two samples was apoAI detected in the urine at 0.5 mg/L. These results have been taken to indicate that significant metabolism of apoAI through glomerular filtration and tubular absorption is unlikely to occur in humans.

Urinary albumin and low molecular weight protein excretion in the nephrotic syndrome--sequential studies during corticosteroid treatment.

In a preliminary investigation into the behaviour of low molecular weight proteins in the nephrotic syndrome, we have measured urinary concentrations of albumin, alpha-1-microglobulin (alpha 1-m) and retinol-binding protein (RBP) in six children for up to 11 days during the course of steroid therapy for nephrotic syndrome. The results in part support the concept of independent proximal tubular absorption of albumin and low molecular weight proteins, and indicate that in the nephrotic syndrome the excretion of RBP and alpha 1-m, two generally accepted markers of tubular proteinuria, is anomalous.

Calibrant solutions for continuous-flow urate analysis: interference by formaldehyde.

The use of formaldehyde to preserve aqueous urate calibration solutions has been recommended since 1913, but it interferes in some commonly used methods. At a pH below 8.5 formaldehyde affects the rate of dialysis of urate in Technicon AutoAnalyzer systems. This effect has been studied by variation of the pH at which dialysis takes place. The interference is attributed to the formation of pH-dependent addition products between uric acid and formaldehyde, which have been examined by thin-layer chromatography and mass spectrometry. Recovery experiments on protein-containing solutions show that a purely aqueous urate calibrant solution is satisfactory for use in AutoAnalyzer systems.

A radioimmunoassay for human urinary prealbumin.

We describe a validated radioimmunoassay for prealbumin in urine. Using timed overnight urine samples, the normal reference range was less than 10-148 micrograms/L; or less than 1.8-9.6 micrograms/mmol creatinine, excretion in women being significantly greater than in men (P < 0.05); or less than 7.3-114 ng/min with no significant difference in excretion rate between the sexes. Urines exhibited loss of immunoreactivity after storage at -20 degrees C and thawing. No such loss occurred after storage for 4 weeks at 4 degrees C or room temperature. The urinary excretion of prealbumin was highly correlated with that of albumin (r = 0.85), and clearance relative to creatinine was 2 x 10(-6), the same order as that of albumin.

Comparison of plasma disappearance and standard clearance techniques for measuring glomerular filtration rate in children with and without vesico-ureteric reflux.

The plasma disappearance rate, or slope clearance, after a single intravenous injection of 51Cr-EDTA was compared with simultaneously determined standard (UV/P) clearances of inulin, creatinine and 51Cr-EDTA in 4 healthy adults and 18 children with renal disease but no reflux. Slope clearance correlated well with standard inulin (r=0.99) and 51Cr-EDTA (r=0.97) clearances and was more accurate than creatinine clearance. Slope clearance x0.82 estimated standard 51Cr-EDTA clearance with +/- 12% accuracy in these subjects. In 18 children with vesico-ureteric reflux the correlations of the standard clearance methods with slope clearance were significantly reduced. Slope clearance may be measured with only two plasma samples and is recommended as the method of choice for determining glomerular filtration rate in children with gross reflux or impaired ability to empty their bladders.

The significance of variation in the selectivity of proteinuria.

The selectivity of proteinuria has been determined immunochemically at least 4 times over periods of 3 years or more in 27 children and adolescents who had been investigated by renal biopsy. Variations of the selectivity outside the limits of experimental error were observed in 14 patients, in 8 of whom there was a progressive decline. Six of these 8 had focal and segmental glomerular lesions, including one case of Alport's syndrome, and 2 had proliferative glomerulonephritis. Two different anomalies of relative IgG clearance were noted: in proliferative glomerulonephritis there was a constantly low clearance, and in focal glomerulosclerosis an elevated clearance increasing with time. Indirect evidence suggests that the latter may be due to the presence of low molecular weight IgG fragments in serum and urine.