Why do nanowires grow with their c-axis vertically-aligned in the absence of epitaxy?

Images of uniform and upright nanowires are fascinating, but often, they are quite puzzling, when the substrate is clearly not an epitaxial template. Here, we reveal the physics underlying one such hidden growth guidance mechanism through a specific example - the case of ZnO nanowires grown on silicon oxide. We show how electric fields exerted by the insulating substrate may be manipulated through the surface charge to define the orientation and polarity of the nanowires. Surface charge is ubiquitous on the surfaces of semiconductors and insulators, and as a result, substrate electric fields need always be considered. Our results suggest a new concept, according to which the growth of wurtzite semiconductors may often be described as a process of electric-charge-induced self-assembly, wherein the internal built-in field in the polar material tends to align in parallel to an external field exerted by the substrate to minimize the interfacial energy of the system.

Neuropsychological function and REM sleep in schizophrenic patients.

To test the hypothesis that rapid eye movement (REM) sleep in schizophrenic patients is associated with cognitive function, we studied 18 schizophrenic inpatients by means of electroencephalograms taken during sleep in their own hospital beds after a minimum 2-wk medication withdrawal period. Patients underwent neuropsychological tests to measure memory function and other aspects of cognitive performance. REM sleep measures demonstrated positive and negative correlations with cognition and memory measures, depending on when REM occurred after sleep onset. Minutes of REM sleep and REM density in the first period correlated negatively with performance, while REM minutes occurring after the first REM period correlated positively with neuropsychological performance. Further work should test whether phasic REM sleep regulation at the beginning of the night plays a compensatory role for neuropsychological dysfunction in schizophrenics.

Modifiability of neuropsychological dysfunction in schizophrenia.

Schizophrenia is often characterized by compromised neuropsychological functioning, especially on tasks sensitive to frontal and temporohippocampal functions but the extent to which cognitive dysfunction can be modified in schizophrenics remains unclear. Twenty-four inpatient schizophrenics and 24 intellectually and demographically matched, inpatient mood-disordered controls were randomly assigned to one of two conditions. Subjects assigned to the cued condition received instructional cues on measures of visual and semantic memory, executive function, and constructional ability. Subjects in the standard condition performed the same neuropsychological measures without cues. The present study revealed some degree of plasticity of neurobehavioral function in schizophrenia.

Effect of instructional cues on schizophrenic patients' performance on the Wisconsin Card Sorting Test.

OBJECTIVE: Schizophrenic patients are particularly deficient on measures of executive functioning, notably the Wisconsin Card Sorting Test. This study was conducted to determine the efficacy of a cuing strategy in facilitating performance on this cognitive measure of the integrity of prefrontal brain structures and functioning. METHOD: Twenty-four schizophrenic inpatients and 24 demographically matched inpatients with mood disorders were administered the Wisconsin Card Sorting Test either with instructional cues at the beginning of the task or with the standard administration procedure. RESULTS: There was a significant benefit of cues for the patients with affective disorders as well as for the schizophrenic patients. The schizophrenic subjects in the uncued condition maintained poor but stable performance throughout the course of the task. CONCLUSIONS: The study suggests that the deficit in executive functioning of schizophrenic patients may lie in the formation of concepts, not in their application.

Neuropsychology of first-episode schizophrenia: initial characterization and clinical correlates.

OBJECTIVE: Neuropsychological impairments are well documented in schizophrenia and are important targets of treatment. Information about the severity and pattern of deficits after treatment for the first psychotic episode and about relationships between these deficits and syndromal characteristics remains limited. METHOD: Comprehensive neuropsychological assessments including 41 individual tests were given to 94 patients with first-episode schizophrenia after initial stabilization of psychosis and to a comparison group of 36 healthy volunteers. Profiles of neuropsychological deficits and the relationship of deficits to sex and handedness were examined. Correlations of neuropsychological deficit with a broad range of historical and clinical characteristics, including outcome, were explored. RESULTS: Patients had a large generalized neuropsychological deficit (1.5 standard deviations compared to healthy volunteers). Patients also had, superimposed on the generalized deficit, subtle relative deficits (less than 0.5 standard deviation compared to their own average profile) in memory and executive functions. Learning/memory dysfunction best distinguished patients from healthy individuals; after accounting for this difference, only motor deficits further distinguished the groups. Patients with higher neuropsychological ability had only memory deficits, and patients with lower ability had both memory and executive deficits. No sex differences were observed beyond the normal advantage for men in motor speed. Dextral patients had less severe generalized deficit. Severity of residual symptoms was associated with greater generalized deficit. Executive and attentional deficits were most linked to global functional impairment and poor outcome. CONCLUSIONS: The results document a large generalized deficit, and more subtle differential deficits, in clinically stabilized first-episode patients. Learning/memory deficits were observed even in patients with less severe generalized deficit, but the pattern was unlike the amnestic syndrome and probably reflects different mechanisms. Executive and attentional deficits marked the more severely disabled patients, and may portend relatively poor outcome. Failure to develop typical patterns of cerebral dominance may increase the risk for greater generalized deficit.

Meta-analysis of symptom factors in schizophrenia.

Factor and correlational analyses have been used to characterize symptom dimensions in schizophrenia, though they have yielded divergent models. This study used meta-analysis of published work to determine the number and composition of symptom dimensions. Principal components analysis of data from 10 empirical studies (pooled n = 896) yielded three factors, 'positive', 'negative' and 'conceptual disorganization'. The findings suggest that a three-factor solution is a relatively stable outcome of studies assessing these symptoms in chronic patients, and that some symptoms (alogia, attentional impairment) are less likely to load uniquely on a single factor.

Comparison of indices of premorbid function in schizophrenia.

There has been a resurgence of interest in the area of premorbid functioning in schizophrenia as it provides clues to onset and etiology. Most studies rely on retrospective estimates of premorbid status that are incomplete, such as the Premorbid Adjustment Scale (PAS). Even prospective high-risk studies are hampered by the narrow range of premorbid functions assessed and are thus unable to answer crucial questions related to onset of illness. This study was undertaken to assess the relationship between several indices of premorbid functioning. Sixty four in-patients with schizophrenia were assessed at medication-free baseline and post-treatment with BPRS and SANS. PAS scores were derived from all available sources. Premorbid cognitive ability was estimated by the mean of WAIS-R Vocabulary and Information subscale scores. Estimated premorbid IQ was obtained using a demographic regression formula. Years of education and predicted VIQ, PIQ, and FSIQ were found to correlate with estimated premorbid cognitive ability. Predicted VIQ, PIQ, and FSIQ were associated with years of education and PAS childhood, early and late adolescence, and general scores. Each estimate of premorbid ability demonstrated a different pattern of association with clinical ratings, symptom change, and outcome. The results suggest that education, PAS, predicted IQ, and WAIS-R estimates of premorbid cognitive ability assess different but overlapping areas of pre-morbid functioning.

Reduced anterior cingulate gyrus volume correlates with executive dysfunction in men with first-episode schizophrenia.

Although frontal lobe structural and functional abnormalities have been identified in schizophrenia, their relationship remains elusive. Because the frontal lobes are both structurally and functionally heterogeneous, it is possible that some measures of frontal lobe structure may not have accurately identified relevant frontal lobe subregions. The authors hypothesized that the volumes of two dorsal, 'archicortical' subregions (i.e. superior frontal gyrus and anterior cingulate gyrus), but not a ventral, 'paleocortical' subregion (i.e. orbital frontal region) would be significantly and selectively correlated with executive and motor dysfunction in patients with schizophrenia as previously reported for the anterior hippocampal region. Volumes of these frontal lobe subregions were measured from magnetic resonance images based on sulcal anatomy in 20 men and 15 women with first-episode schizophrenia. All patients completed a comprehensive neuropsychological test battery while clinically stabilized that encompassed six domains of functioning: attention, executive, motor, visuospatial, memory and language. Findings indicated that reduced anterior cingulate gyrus volume was significantly correlated with worse executive functioning in men; among women, there were no significant correlations. Among men, anterior cingulate gyrus volume was significantly more strongly correlated with executive functioning than with attention, visuospatial, memory, language and general intellectual functioning. Neither executive nor motor functioning was significantly more strongly correlated with the dorsal 'archicortical' volumes than with orbital frontal volume. These findings suggest a link between executive deficits and dysfunction of the dorsal 'archicortical' system and implicate sex differences in their relationship in first-episode schizophrenia.

Validation of the 16-item Negative Symptom Assessment.

The dimensional structure of the 16-item Negative Symptom Assessment (NSA-16) was validated in a sample of 223 unmedicated schizophrenic inpatients and cross-validated on an independent sample of 276 patients with schizophrenia. Using a confirmatory factor analytic procedure, a five factor model was found to best characterize the structure of this rating instrument. These factors include: Communication, Emotion/Affect, Social Involvement, Motivation, and Retardation. The latent structure of the NSA-16 is similar to the larger instrument from which it was derived. The findings provide support for a multidimensional model of negative symptoms in schizophrenia and offer a useful measure for their assessment.

Endothelin induces a sustained rise in intracellular calcium in hippocampal astrocytes.

We report that the endothelins, a newly described family of vasoactive peptides, have a profound effect on intracellular calcium levels of cultured rat hippocampal astrocytes that resembles the effect of endothelin (ET) on vascular smooth muscle cells (VSMCs) in many respects. The astrocyte's response has two components that can be distinguished by their extracellular calcium requirement and time course. Within seconds of application, ET induces a transient calcium spike that corresponds to a release of calcium from internal stores. The second component follows immediately, is dependent upon extracellular calcium, and maintains an elevated intracellular calcium level for many minutes. Sustained elevations of intracellular calcium can dramatically alter astrocyte morphology and induce cell division in many other cell types. ET may serve these functions, and thus form a communication link between blood vessels and neurons through astrocytes.

Factor structure of the negative symptom assessment.

The factor structure of the Negative Symptom Assessment (NSA), a standardized negative symptoms rating scale, was systematically evaluated in a group of 223 inpatients with schizophrenia. Confirmatory factor analyses found that a six-factor model best described the NSA. More specifically, the domains of Communication, Emotion/Affect, Social Involvement, Motivation, Gross Cognition, and Retardation characterized the rating scale. This latent structure of the NSA is consistent with a multidimensional conceptualization of negative symptoms.

The minimal dose of dialysis required for a target KT/V in continuous peritoneal dialysis.

This study attempted to define the minimal dose of dialysis needed to produce a target KT/V in continuous peritoneal dialysis (CPD). In a training set of 143 clearance studies performed in 92 CPD patients, logistic regression identified low urine volume (UV) and low dialysate drain volume normalized by body water (DV/V) as predictors of weekly KT/V urea < or = 1.70. Solution of the regression equation with UV fixed at 0.00 1/24 h and at different probabilities of low KT/V provided a series of minimal DV/V values consistent with weekly KT/V > or = 1.70 in anuria. The accuracy of the logistic regression model and of the DV/V cut-offs was tested in a validation set (VS) of 189 urea kinetic studies performed in another 102 CPD patients. In the VS, the area under the Receiver Operating Characteristic curve generated by the regression model was 0.832 (95% Confidence Interval: 0.798-0.866). The DV/V cut-off value of 0.301 per 24 h, calculated by solving the regression model at p = 0.442 and with UV = 0, identified studies with weekly KT/V < 1.70 with a sensitivity of 89.3% and a specificity of 78.1% in anuric VS subjects (n = 60). Use of only the first urea kinetic study from each patient did not modify the predictors of KT/V or the cut-off values derived from solution of the regression model. The DV/V cut-off of 0.324 per 24 h, derived from the logistic regression model predicting KT/V < or = 1.90, identified KT/V < 1.90 in VS anuric subjects with a sensitivity of 94.3% and a specificity of 81.0%. Low UV and DV/V predict low KT/V urea in CPD. Prescribed 24 h exchange volume in anuric CPD subjects should be calculated to produce DV/V values exceeding 0.301 1/24 h per 1 body water for a KT/V of 1.70 and 0.324 1/24 h per 1 body water for a target weekly KT/V of 1.90.

Experience-dependent cognitive recovery in alcoholics: a task component strategy.

Visuospatial problem-solving deficits following chronic alcohol abuse may not readily recover spontaneously after drinking cessation but may reverse with appropriate environmental stimulation. To determine if such recovery in alcoholics under age 40 may be accelerated by training with components of an initially impaired task (Trails B), this study employed four groups of alcoholics (N = 53) and a group of matched controls (N = 13). Two alcoholic groups received two consecutive cognitive remediation sessions during the latter 2 weeks of a 1-month treatment program, and two groups of alcoholics received no remediation. Results confirmed that recovery of visuospatial problem-solving skills is facilitated by training with task components (experience-dependent recovery) while spontaneous recovery during the first month of abstinence is minimal for this task. These findings demonstrate the efficacy of cognitive remediation in reversing some alcohol-induced cognitive impairment and have important implications for improving alcohol treatment outcome and adaptive functioning.

Creatinine clearance in continuous peritoneal dialysis: dialysis dose required for a minimal acceptable level.

OBJECTIVES: To identify the most advantageous formula for estimating creatinine clearance (CCr) and to establish a dose of dialysis that will ensure minimal acceptable levels of creatinine clearance in patients on continuous peritoneal dialysis (CPD). DESIGN: Analysis of all CCr studies performed in CPD patients over 40 months. SETTING: All four dialysis units following CPD patients in one city. One dialysis unit is government-owned, one is university-affiliated, and two are community based. PARTICIPANTS: One hundred and ninety-four patients representing almost the entire CPD population in Albuquerque. INTERVENTIONS: Creatinine and urea clearance studies were performed in 24-hour urine and drained dialysate samples. Creatinine clearance (peritoneal plus urinary) was normalized to either 1.73 m2 body surface area (CCr) or body water estimated by the Watson formulas (KT/VCr). CCr and KT/VCr were either corrected by averaging urinary creatinine and urea clearances or were not corrected. Two dialysis units were designated as the training set (92 patients, 143 clearance studies) and the other two units as the validation set (102 patients, 181 clearance studies). MAIN OUTCOME MEASURES: Minimal acceptable creatinine clearance levels were determined in the training set by computing the creatinine clearance value corresponding to 1.70 weekly KT/V urea by linear regression. Logistic regression models predicting low creatinine clearance were developed in the training set and were tested in the validation set. RESULTS: The following weekly creatinine clearance values corresponded to 1.70 KT/V urea: corrected CCr 52.0 L/1.73 m2, uncorrected CCr 54.4 L/1.73 m2, corrected KT/VCr 1.46, uncorrected KT/VCr 1.53. Logistic regression identified as predictors of low creatinine clearance low daily urine volume (UV) and low daily dialysate drain volume/body water (DV/V) for all four creatinine clearance formulas, plus low/low-average peritoneal solute transport (only for uncorrected CCr) and serum creatinine (for both KT/VCr formulas). In the validation set, the predictive models produced an area under the receiver operating characteristic (ROC) curve between 0.835 and 0.919 indicating very good predictive accuracy. For corrected CCr and anuria, the regression model produced a minimal normalized drain volume (DV/V) value consistent with minimal acceptable CCr equal to 0.305 L/L per 24 hours. This DV/V cutoff detected low corrected CCR in validation set anuric subjects (n = 55) with a sensitivity of 85% and a specificity of 71%. For uncorrected CCR and anuria, DV/V cutoffs were 0.273 L/L per 24 hours (high/high-average peritoneal solute transport) and 0.420 L/L per 24 hours (low/low-average transport). Sensitivity and specificity of these cutoffs in validation set anuric subjects were 87% and 85%, plus 86% and 33%, respectively. CONCLUSIONS: The uncorrected CCr appears to be the most advantageous creatinine clearance formula in CPD, because it allows the use of peritoneal solute transport type in the calculation of the minimal required normalized drain volume. The minimal acceptable uncorrected CCr is 54.4 L/1.73 m2 weekly. To achieve this uncorrected CCr in anuria, the required minimal normalized drain volume is 0.273 L per liter of body water daily if peritoneal solute transport is high or high-average and around 0.420 L per liter of body water daily if peritoneal solute transport is low or low-average. The required total daily drain volume is computed by multiplying the required normalized drain volume by body water.

Volume of distribution and fractional clearance of urea in amputees on continuous ambulatory peritoneal dialysis.

OBJECTIVES: To demonstrate the effects of amputation on the estimates of urea volume of distribution (V) and KT/V urea in continuous ambulatory peritoneal dialysis (CAPD) patients and to present a method for correcting the errors created by the uncorrected anthropometric formulas estimating V. DESIGN: (1) A mathematical analysis of the error and the correction proposed was performed. (2) Urea kinetic modeling with uncorrected and corrected estimates utilizing both the Watson and the Hume anthropometric formulas was performed in amputees on CAPD. SETTING: Subjects were recruited from four dialysis units in one city: one Veterans Affairs unit, one university-affiliated unit, and two community units. PATIENTS: Fourteen amputees on CAPD:12 with unilateral leg amputation and 2 with bilateral leg amputation, at the same length of the leg, were studied. INTERVENTIONS: Urea kinetic studies were performed in 24-hour drained dialysate and urine specimens. MAIN OUTCOME MEASURES: Uncorrected and corrected estimates of V and KT/V urea were compared to each other and to the predictions of the mathematical model. Body weights corresponding to uncorrected and corrected V estimates were compared to the actual body weights. RESULTS: (1) The mathematical model predicts that uncorrected estimates by the anthropometric formulas will falsely characterize unilateral amputees as leaner than they are and bilateral amputees as more obese than they are. (2) In unilateral amputees studied with the Watson formulas, uncorrected V was 0.546 +/- 0.023 L/kg and corrected V was 0.520 +/- 0.023 L/kg (p < 0.001). Corresponding weekly KT/V urea values were 1.97 +/- 0.14 and 2.07 +/- 0.14, respectively (p < 0.001). Similar results were obtained with the Hume formulas. In bilateral leg amputees studied with the Watson formulas, uncorrected V was 0.479 +/- 0.022 L/kg and corrected V was 0.514 +/- 0.023 L/kg. Corresponding KT/V estimates were 2.11 +/- 0.45 and 1.96 +/- 0.14, respectively. The differences were even greater with the Hume formulas. Estimates of body weight calculated from corrected V values were equal to actual weight measurements, whereas those calculated from uncorrected V values were lower than actual body weight measurements in unilateral amputees, and much higher than actual body weight measurements in bilateral amputees. CONCLUSION: Uncorrected anthropometric estimates falsely characterize unilateral amputees as leaner than they actually are and bilateral amputees, amputated at the same leg length, as more obese than they actually are. Uncorrected KT/V estimates are, therefore, falsely low in unilateral amputees, and falsely high in bilateral amputees. The proposed correction of the anthropometric formulas provides estimates agreeing closely with dietary estimates of body composition. Further studies are needed to define the accuracy of the corrected formulas.

Protein catabolic rate in patients on continuous peritoneal dialysis. A multivariate predictive model.

Protein catabolic rate (PCR) and PCR normalized to standard weight (PCRN) are important indices of nutrition in patients on continuous peritoneal dialysis. The purpose of this study was to test whether urea clearance is among the predictors of PCR and PCRN in a multivariate analysis. Stepwise logistic regression was used to develop separate models for low PCR and low PCRN on a set of 143 urea kinetic studies in 92 patients on continuous peritoneal dialysis. The regression models were tested on an independent sample of 189 urea kinetic studies in 102 patients on continuous peritoneal dialysis by deriving the area under a receiver operating characteristic curve. In the derivation set, low serum urea, high serum creatinine, low urine and dialysate drain volumes, and low body surface area were identified as predictors of PCR < or = 50 g daily. The area under the receiver operating characteristic curve in the validation set was 0.930 (95% confidence interval: 0.915-0.945). Low serum urea, male gender, high body mass index and low urea fractional clearance (KT/V) were predictors of PCRN < or = 0.80 g/kg daily. The receiver operating characteristic area for this model was 0.948 (95% confidence interval: 0.926-0.970). Logistic regression analysis was repeated twice after adding urea nitrogen excretion normalized to standard weight (UNEN) as a candidate variable. This process identified low UNEN, male gender, and obesity as the predictors of low PCRN, and low UNEN, male gender, low urine volume, low drain volume normalized by body water, and high serum albumin as predictors of low KT/V urea. The authors conclude that PCR and PCRN can be predicted by models that incorporate serum azotemic indices, body size and composition, and direct or indirect measurements of urea clearance. Small body size and lean body composition predict low PCR but high PCRN values. Both PCRN and KT/V urea are predicted by UNEN. Multivariate analysis cannot, therefore, rule out the hypothesis that PCRN and KT/V are linked mathematically.

An analysis of the determinants of urinary urea and creatinine clearance in patients on continuous peritoneal dialysis.

The relative contribution of urinary volume (UV) and urine-to-plasma concentration ratios for urea (U/PUr) and creatinine (U/PCr) to urinary Kt/V urea (Kt/VU) and urinary uncorrected creatinine clearance (CCrU), respectively, was studied by simple and multiple linear regression analysis in 236 urea kinetic studies and 233 creatinine kinetic studies performed in 135 patients on continuous peritoneal dialysis (CPD). The following simple regressions were obtained: Kt/VU = 0.09 + 0.72 (UV), r = 0.75; Kt/VU = -0.01 + 0.11 (U/PUr), r = 0.55; CCrU = 12.06 + 56.46 + 46.46 (UV), r = 0.62; CCrU = 3.51 + 3.40 (U/PCr), r = 0.58. All r values were significant (p < 0.001). According to these regressions, a loss of 0.2 L/24 hours in UV leads to a loss of 0.15 weekly in Kt/VU and 11.3 L/1.73 m2 weekly in corrected CCrU (approximately 8 L/1.73 m2 weekly in corrected CCrU). By multiple linear regression, (1) Kt/VU = -0.38 + 0.70 (UV) + 0.10 (U/PUr). Standardized coefficients were 0.72 for UV and 0.51 for U/PUr (2) CCrU = -33.36 + 59.83 (UV) + 3.63 (U/PCr). Standardized coefficients were 0.65 for UV and 0.61 for U/PCr. UV is the most important determinant of both urea and creatinine urinary clearances in CPD patients. The contribution of the U/P ratios to the urinary clearances is important, but less than that of UV. The primary dependence of urinary clearances on UV allows the use of UV, which can be easily monitored by patients, as a first approximation index of changing residual renal function in CPD.

Early and late peritoneal dialysate leaks in patients on CAPD.

Dialysate leaks, which occurred in 386 CAPD patients over 11 years, were analyzed retrospectively. 18 patients developed 21 early leaks (within 30 days of catheter insertion) and 18 patients developed 28 late leaks (beyond 30 days of catheter insertion). 8 patients had multiple (2-6) leaks. Both early and late leaks, particularly if they were multiple, were associated with conditions that affect adversely tissue healing and tensile strength. Median surgical insertion was apparently associated with a higher incidence of early leaks. Early leaks were manifested externally, usually through the exit site, and did not require imaging. Late leaks were manifested usually by poor dialysate outflow, localized edema and subcutaneous fluid collections. One third of the late leaks required radiological imaging. Hernias caused 42% of the late leaks. Early leaks were managed by temporary discontinuation of CAPD alone (57%) or surgery. Most late leaks (67%) required surgery. Conservative means (change to IPD, observation) were applied for the management of a few late leaks. Both early and late leaks resulted frequently in replacement of peritoneal catheters, but only late leaks resulted in permanent discontinuation of peritoneal dialysis. Paramedian surgical insertion, waiting period of 10-14 days between catheter insertion and initiation of CAPD, and low starting dialysate volumes have resulted in apparent reduction of the incidence of the early, but not of the late leaks. Dialysate leaks have serious consequences on the performance of CAPD. Early leaks differ from late leaks in some clinical manifestations. Preventive measures have decreased the incidence of early, but not of the late leaks.

Cross-sectional analysis of erythropoietin use in CPD: its relation to azotemic index clearances.

The association between the use of erythropoietin and urea or creatinine clearance was studied in two populations on continuous peritoneal dialysis (CPD) residing either at an altitude of 1600 m (n = 194) or at sea level (n = 108). Among peritoneal and total KT/V urea and creatinine clearance (CCr) indices, only total CCr was lower in the high altitude group receiving erythropoietin than in the corresponding group not receiving erythropoietin (68.0 +/- 34.9 vs 82.9 +/- 40.9 L/1.73 m2 weekly, p < 0.01). However, 24-hour urine volume and urinary KT/V urea and CCr were consistently lower in the groups receiving erythropoietin than in those not receiving erythropoietin. Total weekly KT/V urea < or = 1.70 and CCr < or = 52 L/1.73 m2 were considered indicators of inadequate CPD. Although the percent of patients receiving erythropoietin did not differ overall between groups with adequate and those with inadequate CPD, a trend towards more frequent use of erythropoietin was found in the sea level group with inadequate CCr versus the group with adequate CCr (28.2% vs 16.9%, p = 0.084). In CPD decreased renal function is associated with more frequent use of erythropoietin. Whether inadequate total urea or creatinine clearance is also associated with more frequent erythropoietin use requires further study.