Essentials of laboratory medicine for the nephrology clinician

Polyuria in adults. A diagnostic approach based on pathophysiology

Polyuria is a common clinical condition characterized by a urine output that is inappropriately high (more than 3 L in 24 h) for the patient's blood pressure and plasma sodium levels. From a pathophysiological point of view, it is classified into two types: polyuria due to a greater excretion of solutes (urine osmolality >300 mOsm/L) or due to an inability to increase solute concentration (urine osmolality <150 mOsm/L). Sometimes both mechanisms can coexist (urine osmolality 150-300 mOsm/L). Polyuria is a diagnostic challenge and its proper treatment requires an evaluation of the medical record, determination of urine osmolality, estimation of free water clearance, use of water deprivation tests in aqueous polyuria, and measurement of electrolytes in blood and urine in the case of osmotic polyuria.

Longitudinal study of bone mineral density in children with idiopathic hypercalciuria

Children with idiopathic hypercalciuria (IH) may have a reduced bone mineral density (BMD), which could impact on bone health in adulthood. There is currently no strong evidence for a preferred treatment of such children. The aim of our study was to evaluate the BMD z-score before and after treating children and adolescents with IH with potassium citrate and thiazides. The study consisted of a historical cohort of 80 pediatric patients who were evaluated between October 1989 and November 2010. Bone scanning and densitometry measurements were made with dual-emission X-ray absorptiometry. Lumbar-spine BMD (g/cm(2)) and BMD z-score were evaluated before and after treatment. The t test and Mann-Whitney U test were used for statistical analysis. Forty-three boys and 37 girls were followed for a median time of 6.0 years. Median calcium excretion before and after treatment was 5.0 and 2.6 mg/kg/24 h, respectively. The BMD z-score changed significantly from -0.763 ± 0.954 (mean ± SD) to -0.537 ± 0.898 (p < 0.0001) before and after treatment, respectively. The BMD z-score of the patients improved with treatment, suggesting a beneficial effect and potential need for treatment. However, the lack of a control group points to the need for future studies to corroborate this outcome.

Impact of standardization of creatinine methodology on the assessment of glomerular filtration rate in children

There is a global effort to standardize clinical laboratory serum creatinine measurements to the reference method of isotope-dilution mass spectrometry (IDMS). Creatinine values in serum and urine are frequently used in children to calculate creatinine clearance (mCrCl) or estimate glomerular filtration rate (GFR) by Schwartz's equation (eGFR). The original normative data of mCrCl and eGFR were developed using Jaffe method. To investigate what impact the differences in methodologies of creatinine analysis will have on mCrCl and eGFR, we measured creatinine in random serum and urine samples by three commercially available assays: Jaffe (J), enzymatic (E) and enzymatic method traceable to IDMS (E-IDMS). There was a significant bias in the two enzymatic methods when compared with J method. The theoretical predicted errors in overestimating mCrCl ranged from 1.10 to 1.34 by E and 1.20 to 1.54 by E-IDMS; and in calculating eGFR 1.07-1.16 by E and 1.30-1.46 by E-IDMS, which was further confirmed in children who had formal GFR evaluation. Thus, as the clinical laboratories calibrate their creatinine assays to the gold standard IDMS method, it is important for the pediatric nephrology community to develop new equations for estimation of GFR based on the new creatinine assay.

Monitoring renal function and limitations of renal function tests

Chronic kidney disease (CKD) is a world-wide public health problem, with adverse outcomes of kidney failure, cardiovascular disease, and premature death. The National Kidney Foundation, through its Kidney Disease Quality Outcome Initiative (K/DOQI) and other National institutions, recommend glomerular filtration rate (GFR) estimates for the definition, classification, screening, and monitoring of CKD. Prediction equations based on serum creatinine values were chosen both for adults (Cockcroft-Gault [C-G] and Modification of Diet in Renal Disease [MDRD] study equations) and for children (Schwartz and Counahan-Barratt equations). This review aims to evaluate from recent literature the clinical efficiency and relevance of these equations in terms of bias, precision, and reproducibility in different specific indications (eg, screening CKD, assessment of disease progression, or therapy efficacy) in different populations. Because these prediction equations based on serum creatinine have limitations, especially in the normal or near-normal GFR range, kidney transplant recipients, and pediatric populations, other prediction equations based on serum cystatin C value were also considered as possibly more sensitive GFR surrogate markers. Recent guidelines state that the cystatin C-based prediction equation cannot be recommended for use in clinical practice. With prediction equations based on serum creatinine, the National Kidney Disease Education Program (NKDEP) recommendations are to report a numerical estimate in round numbers only for GFR values <60 mL/min per 1.73 m(2). The MDRD equation generally outperforms the C-G equation but may still have a high level of bias, depending on creatinine assay calibration, and low precision with, at best, approximately 80% of estimated GFR in the "accuracy range" of 70-130% of the measured GFR value, even in patients with known CKD. According to Kidney Disease Improving Global Outcomes (KDIGO) recommendations, many indications remain for GFR measurements using a clearance method. In that context, it should be recalled that radiolabeled-tracer plasma or urinary clearance methods, are safe, simple, accurate and reproducible.