1 Department of Emergency Medicine and 2 Clinic of Dermatology, University of Vienna, Vienna, Austria
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Abstract |
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Methods. Following collection of a spot, midstream urine sample, urine was collected for 24 h for the measurement of microalbumin in 264 hypertensive patients. We compared microalbumin concentration in the spot urine with microalbumin measured in the 24-h urine sample and examined the utility of the ACR in evaluating microalbuminuria in hypertensive patients. Pathologic microalbuminuria was assumed when the microalbumin concentration exceeded 30 mg/l in the 24-h urine sample. Diagnostic performance is expressed in terms of specificity, sensitivity, positive (PPV) and negative predictive value (NPV), and area under receiver operating characteristics curve (AUC).
Results. A total of 47 samples (17.8%) showed pathologic microalbuminuria in the 24-h urine sample. The diagnostic performance expressed as AUC was 0.94 (95% CI 0.900.98) for microalbumin measurement alone and 0.94 (95% CI 0.890.97) for ACR. The PPV and NPV were 44.2 and 97.9% for microalbumin measurement alone. ACR revealed a PPV of 29.3% and a NPV of 96.2% for males and 42.9 and 98% for females, if a cut-off value of 2.5 mg/mmol for males and of 4.0 mg/mmol for females was used.
Conclusions. The ACR did not provide any advantage compared with microalbumin measurement alone, but requires an additional determination of creatinine and the use of gender-specific cut-off values. Therefore, measurement of microalbuminuria alone in the spot urine sample is more convenient in daily clinical practice and should be used as the screening method for hypertensive patients.
Keywords: microalbuminuria; sensitivity; specificity; spot urine sampling
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Introduction |
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Proteinuria is usually detected by using dipstick urinalysis. However, recent studies have documented the inaccuracies of this method, giving high false positive and negative results when compared with the gold standard of 24-h urine measurement [4,5]. An alternative method is the spot urine sample, which can easily be acquired in the ambulatory setting. This method has certain advantages, as it is easily available, non-expensive, and reproducible. The results of the spot urine sample may have diagnostic and therapeutic consequences in hypertensive patients. The best method to diagnose microalbuminuria in the spot urine sample is still a matter of discussion. It remains unclear whether microalbumin measurement alone is a sufficient screening method or a calculation of an albumin/creatinine ratio (ACR) is required to detect microalbuminuric hypertensive patients. Recently, published data have shown that the calculation of an ACR exhibited a higher sensitivity and specificity in the detection of microalbuminuria in different groups of highly selected patients.
We therefore evaluated whether microalbumin measurement alone or calculation of the ACR are more predictive for the detection of microalbuminuria in non-selected hypertensive patients.
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Patients and methods |
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Patient's selection
Primary inclusion criterion was the evidence of arterial hypertension defined by 24-h ambulatory blood pressure measurement according to the criteria of the PAMELA study (mean systolic blood pressure >130 mmHg and/or mean diastolic blood pressure >81 mmHg, [6]). Prior to 24-h urine collection, a urine sediment was performed to evaluate leukocyturia or erythrocyturia. Patients with a pathologic urine sediment were primarily excluded from the study and told to admit to the ambulance 710 days later again. The urine sediment was repeated and in case of persistence of an abnormal finding the patient was excluded from the study. Further exclusion criteria were evidence of impaired renal function (defined as serum creatinine >1.5 mg/dl), history of renal transplantation, evidence of a chronic liver disease, congestive heart failure NYHA class II and higher, fever (>38°C), and diabetes (fasting blood glucose >110 mg/dl or HbA1c >6%). Patients were instructed to collect urine during a usual working day without any planned extraordinary events. If the patients did extensive exercise during the collection period, the urine sampling was repeated on another occasion due to the impact of heavy exercising on protein excretion [7,8].
Urine sampling and analysis
Following collection of a spot, midstream urine sample usually, but not always obtained in the morningurine was then collected for 24 h for measurement of microalbumin. Correctness of urine sampling was obtained by measurement of urine volume and calculation of the creatinine clearance. Samples with a urine volume <1000 ml and/or a creatinine clearance <80 ml/min were excluded from further analysis.
The microalbumin concentration measured in the spot urine sample was compared with the microalbumin concentration quantified from 24-h urine sampling using an immunoassay (TINA quant Albumin, Roche Diagnostics GmbH, Basel, Switzerland).
Statistical analysis
Data are given as mean and the standard deviation (SD) unless otherwise specified. Diagnostic performance is expressed in terms of specificity, sensitivity, and areas under receiver operating characteristics curves (AUC). The agreement of spot urine samples and 24-h urine samples was also evaluated using kappa statistics. For the analysis of agreement using kappa statistics the measurements were divided into pathologic and physiologic samples. The cut-offs used for this stratification were as follows: (i) 30 mg/l for the microalbumin measurements in the spot urine; (ii) 30 mg/24 h for the microalbumin measurements in the 24-h urine; (iii) 3.0 mg/mmol ACR for microalbuminuria in the spot urine sample; and (iv) gender-specific values for the ACR, i.e. 2.5 mg/mmol for males and 4.0 mg/mmol for females. Regression analysis and Spearman rank correlation were performed to evaluate the relationship of microalbumin in spot urine and 24-h urine samples. A logarithmic transformation of the raw data was performed to meet the requirements of regression analysis. The agreement of spot urine samples and 24-h urine samples for the detection of pathologic microalbuminuria was also evaluated using kappa statistics.
A P-value of <0.05 was regarded to indicate statistical significance.
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Results |
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Microalbuminuria
To evaluate the diagnostic performance of the microalbumin measurements in the spot urine sample, pathologic microalbuminuria was assumed when the microalbumin concentration exceeded 30 mg/24 h in the 24-h urine sample. The diagnostic performance of the microalbumin measurements in the spot urine sample expressed as AUC was 0.94 (95% CI 0.900.98; Figure 1). The specificity and sensitivity of the spot urine sample at various cut-off levels is shown in Table 1
. Sensitivity and specificity for the recommended threshold of 30 mg/l were 91.5 and 84.3%, respectively. This was the cut-off level with the highest sensitivity and specificity. The positive (PPV) and negative predictive values (NPV) at this level were 44.2 and 97.9%, respectively.
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Albumin/creatinine ratio (ACR)
The overall diagnostic performance of the ACR of the spot urine sample expressed as AUC was 0.94 (95% CI 0.890.97). This diagnostic performance was statistically not different from the crude microalbumin measurements in the spot urine (P=0.73).
When the ACR was evaluated separately for males and females, the AUC was 0.95 (95% CI 0.910.99) for males and 0.95 (95% CI 0.871.00) for females. Using the recommended cut-off level of 2.5 mg/mmol for males, the sensitivity was 87.9% and the specificity was 89.3%. The PPVs and NPVs at this level were 29.3 and 96.2%, respectively. Using the recommended cut-off level of 4.0 mg/mmol for females, the sensitivity was 85.7% and the specificity was 91.4%. The PPVs and NPVs at this level were 42.9 and 98.0%, respectively. Table 2 summarizes the sensitivity and specificity of the ACR at different cut-off levels for males and females.
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Discussion |
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Urinary albumin measurement alone
The recommended threshold of 30 mg/l for microalbumin in the spot urine sample exhibited the highest diagnostic performance, i.e. the value with highest sensitivity and specificity. Moreover, only 2.1% of all patients with microalbuminuria would be missed, if 30 mg/l is used as the cut-off value. This low percentage of patients would be falsely classified as low-risk patients and may not receive appropriate antihypertensive treatment to prevent further end-organ damage. We, therefore, assume that in a non-selected cohort of hypertensive patients the method fulfils the criteria of a screening test, as the NPV of 97.9% revealed a high rate of correctly diagnosed normoalbuminuric patients.
In contrast, the PPV of microalbumin measurement in the spot urine sample was rather low indicating a considerable percentage of falsely positive results.
One important contributor to false positive results is the circadian rhythm of proteinuria [1214]. Koopman et al. [14] demonstrated that proteinuria estimated from a single spot urine sample could be as low as 19% or as high as 349% of actually measured 24-h proteinuria.
We have tried to minimize the effect of this variability by collecting urine samples always in the time between 08:00 to 10:00, when proteinuria is at its lowest level. Nevertheless, evidence of microalbuminuria provided by microalbumin measurement in the spot urine sample always requires confirmation by 24-h urine sampling due to the high rate of false positive results.
Albumin/creatinine ratio (ACR)
The value of ACR is controversially discussed in the recently published literature. Whereas some authors recommended ACR as the method of choice for the detection of microalbuminuria, other published data demonstrated only a limited value of ACR as a screening method for microalbuminuria [9,15]. Our data did not provide any advantage of ACR compared with microalbumin measurement alone, as the overall sensitivity and specificity was not significantly different from the direct measurement of microalbumin in the spot urine. However, sensitivity and specificity of ACR in our study were similar to results published previously, as the reported sensitivity of ACR varied from 73 to 92% (our data: females 6%, males 88%) and the specificity from 90 to 97% (our data: females 91%, males 89%). We, therefore, assume that our data are valid and comparable with historical data.
We also used different cut-off values of the ACR for males and females as recommended previously [16]. However, the NPVs were similar compared with microalbumin measurement alone and did not show any improvement in the number of true negative results in our study population.
ACR also showed no improvement concerning the rate of false positive results compared with microalbumin measurement alone as the PPV for males and females was rather low. The circadian rhythm of proteinuria is the main contributor to the inaccuracy of ACR [14]. The additional determination of creatinine did not improve the rate of true positive results as the creatinine excretion showed only minimal circadian changes during 24 h. Therefore, each change of protein excretion due to the circadian pattern influences the accuracy of the ACR.
In summary, a positive result of the ACR also needs confirmation by 24-h urine sampling. Our results may be in contrast to recently published data, which demonstrated a higher sensitivity of ACR compared with microalbumin measurement alone [9], but no significant differences between both methods concerning specificity [9,17]. We assume that this small improvement of sensitivity has no important clinical impact, as the rate of false positive results remains considerably high.
The impact of a screening test has to be evaluated with regard to the prevalence of the disease in the investigated population. The prevalence in our study population was 17.8%, which was comparable with other epidemiological studies reporting a prevalence of 5.126.2% [1823]. We, therefore, assume that our results are clinically relevant and demonstrate that determination of microalbumin in the spot urine sample is a reliable screening method, as both methods, i.e. microalbumin measurement alone and ACR, provide a high rate of true negative results. However, both methods failed to be useful in the diagnosis of patients with microalbuminuria.
In summary, as ACR did not provide any advantage in terms of positive or negative prediction and requires a higher laboratory effort, microalbumin measurement alone is the more convenient screening method in daily clinical practice.
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Notes |
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References |
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