Abstract
ABSTRACT. Previous retrospective research suggests that low-level environmental lead exposure is associated with an acceleration of age-related impairment of renal function. For elucidating the long-term relationship between low-level environmental lead exposure and progression of chronic renal diseases in patients without diabetes, 121 patients who had chronic renal insufficiency, a normal body lead burden (BLB), and no history of exposure to lead were observed prospectively for 48 mo. Associations of both BLB and blood lead level (BLL) with renal function were evaluated, with reference to other covariates. The primary end point was an increase in the serum creatinine level to double the baseline value. Sixty-three patients had BLB ≥80 μg and <600 μg (high-normal group), and 58 patients had BLB <80 μg (low-normal group). The primary end point occurred in 17 patients. Fifteen of them had high-normal BLB, whereas two patients had low-normal BLB (hazard ratio [95% confidence interval]: 1.01 [1.00 to 1.01] for each increment of 1 μg; P = 0.002). The BLB and BLL at baseline were the most important risk factors to predict progression of renal insufficiency. Each increase of 10 μg in the BLB or 1 μg/dl in the BLL reduced the GFR by 1.3 (P = 0.002) or 4.0 ml/min (P = 0.01) during the study period. In conclusion, low-level environmental lead exposure is associated with accelerated deterioration of renal insufficiency. Even at levels far below the normal ranges, both increased BLL and BLB predict accelerated progression of chronic renal diseases.
A high occupational lead exposure is well documented to be able to induce nephropathy (1,2⇓). Several studies have indicated a strong association between blood lead levels (BLL) and age-related decline in renal function of the general population (3–6⇓⇓⇓). However, these studies either were retrospective or did not adjust other confounding factors that affect the progression of renal function, such as hypertension, urinary protein excretion, and usage of angiotensin-converting enzyme (ACE) inhibitors.
Furthermore, most of these studies measured BLL as an indicator of lead exposure. However, the BLL reflects recent lead exposure rather than the actual body lead burden (BLB). Calcium disodium EDTA mobilization tests and bone x-ray fluorescence studies are the most reliable methods for measuring the BLB (7). A BLB of >600 μg (2.9 μmol), as assessed by calcium disodium EDTA mobilization tests, indicates lead poisoning. The authors’ previous studies, using EDTA-mobilization tests to assess the BLB, suggested that low-level environmental lead exposure may be associated with the progression of renal insufficiency in patients without known lead exposure (8–10⇓⇓). Our recent work further established that repeated chelation therapy to reduce the BLB may slow the progression of renal insufficiency in a 27-mo clinical trial (11). However, the long-term relationship between low-level environmental lead exposure and the progression of chronic renal diseases remains unknown. A 48-mo prospective longitudinal study was conducted to determine whether chronic low-level environmental lead exposure affects the progression of chronic renal diseases in patients without diabetes or occupational lead exposure.
Materials and Methods
Subjects
The Medical Ethics Committee of Chang Gung Memorial Hospital in Taipei, Taiwan, approved the protocol, and all patients provided written informed consent. Patients who were between 25 and 82 yr of age and had chronic renal insufficiency were eligible when they had a serum creatinine concentration between 1.5 mg/dl (132.6 μmol/L) and 3.9 mg/dl (344.8 μmol/L), with a decrease in GFR of <5 ml/min over a period of at least 6 mo; BP <140/90 mmHg; a cholesterol level <240 mg/dl (6.21 mmol/L); a daily protein intake <1 g/kg body wt; and no known history of exposure to lead or other heavy metals (BLB <600 μg [2.90 μmol], as measured by EDTA mobilization testing and 72-h urine collection). Specific renal diagnoses were based on the patients’ history and laboratory evaluations, renal imaging, and renal histologic examination (12).
The exclusion criteria were renal insufficiency with a potentially reversible cause, such as malignant hypertension, urinary tract infection, hypercalcemia, or drug-induced nephrotoxic effects; systemic diseases, such as connective-tissue diseases or diabetes; use of drugs that might alter the course of renal diseases, such as nonsteroidal anti-inflammatory agents, steroids, or immunosuppressive drugs; rapidly progressive glomerulonephritis or a high level of 24-h urinary protein excretion (>8 g/d); previous marked exposure to lead (lead poisoning or occupational exposure); drug allergies; and absence of informed consent.
BP was controlled with diuretics and ACE inhibitors, with or without nondihydropyridine calcium-blocking agents. Angiotensin-receptor antagonists were used when patients could not tolerate ACE inhibitors. Patients with normal BP were not prescribed ACE inhibitors or angiotensin-receptor antagonists. BP, cholesterol, and protein intake were controlled effectively in all patients. Calcium carbonate was prescribed to control phosphate levels. No patients received vitamin D3 supplements or erythropoietin treatment. The patients received dietary consultations, and a normal-protein diet (0.8 to 1.0 g protein/kg body wt per day, provided by foods such as meat, fish, chicken, and eggs) was recommended. A nutritionist reviewed the diet of each patient every 3 to 6 mo and 24-h urea excretion was measured every 3 months to determine nitrogen balance and dietary compliance (13).
Measurements of BLL and BLB
BLL and BLB were measured as described previously (9–12⇓⇓⇓). BLB was measured by EDTA-mobilization tests developed by Emmerson and modified by Behringer et al. (14). BLL and BLB were assessed by 72-h urinary lead excretion after the intravenous infusion of 1 g of calcium disodium EDTA (edetate calcium disodium [Calcium Disodium Versenate]; Abbott Laboratories). They were measured by electrothermal atomic-absorption spectrometry (model 5100 PC; Perkin-Elmer, Norwalk, CT), with a Zeeman background correction and a L’vov platform. Internal and external quality-control procedures yielded consistently satisfactory results (11).
Study Protocol
A total of 140 patients entered the study, and 19 patients who met the exclusion criteria were excluded during the initial 6-mo sample collection period. Finally, 121 patients were enrolled in this study and observed prospectively for 48 mo. Baseline BLL, hemoglobin levels, and BLB were obtained in the beginning of this study. Serum creatinine, blood urea nitrogen, cholesterol, urinary protein, creatinine, and urea excretion were measured every 3 mo using an autoanalyzer system (Hitachi). Urinary excretion measurements were averages of values from two consecutive 24-h urine collections. Renal function was evaluated by measuring creatinine clearance and estimating the GFR (both in milliliters per minute per 1.73 m2 of body surface area) (15).
Outcome Measures
The primary end point was an increase in serum creatinine to double the baseline value or the need for hemodialysis during the longitudinal observation period. A secondary end point was a change in the creatinine clearance or GFR during the study period.
Statistical Analyses
The sample size was calculated with PASS software (power analysis and sample-size package; NCSS Statistical Software). For a two-sided test at the 0.05 significance level, a sample size of 64 patients (32 in each group) would be sufficient to permit the study to detect a difference between both groups in the rate of change in the GFR of 0.31 ml/min per 3-mo interval, with a power of 0.95. The Cox proportional-hazards model was used to determine the significance of the variables in predicting the primary end point during the observation period. This model incorporates all variables related to the progression of renal insufficiency in the literature (5,6⇓). Kaplan-Meier analysis was used to determine the significance of the difference between patients with a BLB of >80 μg and those with a BLB of <80 μg in reaching the primary end point. The comparison was made by conducting the log rank test. The correlation between BLB and BLL was analyzed using linear regression analysis. In addition, generalized estimating equations were applied in longitudinal multivariate analyses using SAS statistical software (version 8.1; SAS; Cary, NC) to investigate further whether a predictor was associated with the progression of renal insufficiency in the studied subjects during the observation period. The interactions among the confounding factors were taken into analysis. The Mann-Whitney U test was used for data that were not normally distributed. All P values were two tailed, and all results are presented as means ± SD. P < 0.05 was considered to be statistically significant.
Results
Characteristics of Study Subjects
A total of 121 patients were observed prospectively for 48 mo. At the baseline, the patients’ mean age was 57.0 ± 13.0 yr (range, 25 to 82); their body mass index (the weight in kilograms divided by the square of the height in meters) was 25.9 ± 3.6 (range, 17.7 ± 35.0); their serum creatinine level was 2.1 ± 0.5 mg/dl (185.6 ± 44.2 μmol/L, with a range of 1.5 to 3.7 mg/dl [132.6 to 327.1 μmol/L]), the creatinine clearance rate was 41.5 ± 11.0 ml/min per 1.73 m2 body surface area (range, 19.2 to 65.9), the estimated GFR was 36.0 ± 9.8 ml/min per 1.73 m2 body surface area (range, 13.6 to 56.4), the daily protein excretion was 0.88 ± 1.13 g (range, 0.03 to 7.20 g), the daily protein intake was 0.94 ± 0.30 g/kg (range, 0.32 to 2.01), the BLL was 4.2 ± 2.2 μg/dl (0.18 ± 0.09 μmol/L; range, 1.0 to 13.4 μg/dl [0.04 ± 0.57 μmol/L]), and the BLB was 99.1 ± 83.4 μg (0.43 ± 0.36 μmol; range, 2.5 to 530 μg [0.01 to 2.27 μmol]). Twenty-six (21.5%) patients had hyperlipidemia. Ninety-two (76.0%) patients had hypertension, which was treated with ACE inhibitors or angiotensin-receptor antagonists in 81 (66.9%) patients. Twelve (9.9%) patients smoked.
Underlying renal diseases included chronic glomerulonephritis in 58 (47.9%) patients, chronic interstitial nephritis in 20 (16.5%), hypertensive nephropathy in 16 (13.2%), polycystic kidney disease in nine (7.4%), obstructive uropathy in two (1.7%), and unknown diseases in 16 (13.2%). At the end of the 48-mo study period, the serum creatinine level was 3.0 ± 1.6 mg/dl (265.2 ± 141.4 μmol/L; range, 1.3 to 10 mg/dl [114.9 to 884.0 μmol/L), the creatinine clearance rate was 31.0 ± 13.6 ml/min per 1.73 m2 body surface area (range, 6.3 to 65.1), and the estimated GFR was 26.6 ± 11.9 ml/min per 1.73 m2 body surface area (range, 4.3 to 60.0).
More Patients with High-Normal BLB Reached the Primary Endpoint: Doubling of Serum Creatinine or Need for Dialysis
On the basis of previous work (10), a high-normal BLB was defined as at least 80 μg (0.39 μmol) and <600 μg (2.90 μmol) of lead. Among the 121 patients, 63 patients had a high-normal BLB. Table 1 presents the baseline characteristics of patients in the groups with a low-normal or high-normal BLB at the beginning of the study. The patients with high-normal BLB (148.0 ± 88.6 μg [range, 80 to 530]) also had higher BLL (4.9 ± 2.6 μg/dl [range, 1.0 to 13.4]) than the patients with low-normal BLB (BLB: 45.8 ± 23.9 μg [range, 2.5 to 79], P < 0.0001; blood lead: 3.4 ± 1.3 μg/dl [range, 1.2 to 6.3], P = 0.0002). The patients in the two groups differed on only BLL and BLB and none of the other baseline characteristics (Table 1).
Table 1. Baseline characteristics of patients in the groups with low-normal or high-normal BLB at the start of the studya
Seventeen patients reached the primary end point, including one patient who needed hemodialysis in the high-normal group. Fifteen of them had high-normal BLB, and two had low-normal BLB. Kaplan-Meier analysis demonstrated that significantly more patients in the group with high-normal BLB reached the primary end point than patients in the group with low-normal BLB (P = 0.001; Figure 1). Cox multivariate regression analysis also indicated that the BLB, instead of other covariates, was the only significant risk factor in the progression of renal insufficiency (hazard ratio [95% confidence interval]: 1.01 [1.00 to 1.01] for each increment of 1 μg; P = 0.002; Table 2).
Figure 1. Kaplan-Meier plots of the primary end point (cumulative survival) for subjects with low-normal (▴) and high-normal (•) lead burden, showing that significantly more patients with high-normal body lead burden (BLB) reached the primary end point (P = 0.001).
Table 2. Cox regression analysis of the overall risk of the primary outcome of progressive renal insufficiency, according to baseline prognostic factorsa
BLB and BLL Are the Most Powerful Predictors of Progressive Decline in Renal Function
Longitudinal multivariate analyses using generalized estimating equations were performed to investigate further the role of low-level environmental lead exposure in the progression of chronic renal disease. The BLB was significantly correlated with the BLL in the patients studied herein (r = 0.47, P < 0.0001; Figure 2).
Figure 2. Plots of linear regression analysis of the correlation between BLB and blood lead levels (BLL), showing that the BLB was positively correlated with the BLL (BLB [μg] = 25.2 + 17.7 × BLL [μg/dl]; r = 0.47, P < 0.0001).
The BLB was found to be the most significant predictor of the progressive decline in the GFR (P = 0.002), whereas male gender (P = 0.06) and the presence of chronic interstitial nephritis (P = 0.05) were borderline-significant predictors of a progressive increase in the GFR (Table 3). Instead, an analysis of the BLL and other factors revealed that the BLL was still the strongest predictor of progressive decline in the GFR (P = 0.01), whereas the presence of chronic interstitial nephritis (P = 0.03) predicted a progressive increase in the GFR (Table 4).
Table 3. Longitudinal analysis of BLB and other predictors of progressive change in the GFR, using generalized estimating equations, during the 48-month longitudinal study period
Table 4. Longitudinal analysis of BLL and other predictors of progressive change in the GFR, using generalized estimating equations, during the 48-month longitudinal study period
Specifically, each increase of 10 μg (0.05 μmol) in the BLB led to a decrease in the GFR by 1.3 ml/min per 1.73 m2 body surface area during the study period (P = 0.002), whereas each increase of 1 μg/dl in the blood lead level reduced the GFR of 4.0 ml/min per 1.73 m2 body surface area during the period of study (P = 0.01), after adjustment for other factors (Tables 3 and 4⇑).
Discussion
The results of the present study indicate that BLB and BLL, even at low levels, are important risk factors in the progression of renal insufficiency. These associations were strong, dose dependent, and consistent, even after comprehensive adjustments had been made for other covariates. The present prospective study is consistent with a previous retrospective cohort study that showed, after adjustment of other co-factors, that the age-related increase in serum creatinine level was earlier and faster in the population with the highest-quartile levels of long-term lead exposure (3).
In comparison with our previous work (11), this study enrolled a new study cohort that was not included in previous studies and presents several new observations. First, after long-term follow-up for 48 mo, only BLB and BLL remained the most important predictors of progressive deterioration of renal function, whereas high baseline serum creatinine level, high daily urinary protein excretion, and underlying renal diseases were no longer significant after adjustment for other factors. Second, the influence of BLB on progressive renal insufficiency was more evident after long-term follow-up; that is, each increase of 10 μg (0.05 μmol) in the BLB reduced the GFR by 1.3 ml/min per 1.73 m2 body surface area during the study period (P = 0.002), which is 43 times the rate published previously. Third, it is important to note that no safe limits of lead indices are noted in the current work. The mean BLL of the study participants was only 4.2 μg/dl, whereas the mean BLB was 99.1 μg—those are less than the data in our previous work and far below the upper limit of the normal range (BLL, 20 μg/dl [0.97 μmol/L]; BLB, 600 μg [2.90 μmol]) (16). Fourth, the present study suggests that environmental lead exposure is a neglected and potentially preventable factor that plays an important role in the progression of chronic renal diseases even though other treatable factors are under control in the long-term follow-up. Last, this work demonstrates that besides BLB, BLL is a powerful predictor of progressive renal insufficiency and can be easily monitored in clinical practice.
Consistent with our study, previous studies of large populations indicated a high correlation between measured BLL and BLB (17,18⇓). If the environmental lead exposure is constant, then higher BLL would be reasonably expected to correlate with higher chronic BLB. This study suggests that BLB is a strong predictor of the progression of renal insufficiency, and measured BLL also would be a good reference for evaluating the risk of progressive renal insufficiency and hence to apply possible lead chelation therapy in clinical practice.
A recent study found that in the population of the United States with hypertension, exposure to lead, even at low levels, is associated with chronic kidney disease (4). However, the study did not measure the BLB. Given the high prevalence of hypertension among people with chronic renal disease, the present prospective survey provides further clues about the interaction among hypertension, BLB, and chronic renal disease. This current work demonstrates that BLB, rather than the presence of hypertension per se, was a significant predictor of progressive renal insufficiency, after adjustment for other factors. Once the BP was controlled in an acceptable range, the BLB may have become the predominant risk factor of progressive renal insufficiency.
The low prevalence (4 of 121) of nephrotic range of proteinuria in our population might partly explain why the daily protein excretion did not predict progression of renal insufficiency. Whether the use of ACE inhibitors or angiotensin receptor blockers would account for the reason that baseline serum creatinine and daily protein excretion did not predict progression of renal insufficiency remains an open question.
Whether increased BLB is the cause or consequence of chronic renal insufficiency has been an issue of debate (19). However, most studies in this area suggest that higher BLL and BLB are the cause, rather than the result, of reduced renal function (17,20–22⇓⇓⇓). Other studies also support the finding that renal dysfunction per se does not increase BLL (4,20,23,24⇓⇓⇓). Chemical and histologic studies of transiliac biopsies obtained from 153 dialysis patients showed that chronic renal failure and dialysis do not cause accumulation of lead in bone (25). Moreover, the present longitudinal work demonstrates that after adjustment of other factors, including the renal function at baseline, both BLB and BLL are the most significant predictors of the progression of renal insufficiency.
Creatinine clearance and the estimated GFR were used as indicators of renal function, given constraints on resources that prevented the use of inulin or isotopic clearances (26). An estimation of the GFR indicated a strong association with isotopic GFR (15). The combination of the assessment of creatinine clearance and estimated GFR provided a better estimate of progressive changes of renal function. Further study with isotope clearance is needed to confirm our results.
The mechanism of lead-induced nephropathy remains unclear. Chronic low-level lead exposure was shown to result in marked hypertension coupled with increased reactive oxygen species production and decreased urinary nitric oxide excretion (27). Furthermore, lead-chelation therapy may reduce the levels of reactive oxygen species associated with nitric oxide inactivation and thus enhance the availability of nitric oxide to the vascular smooth muscle, potentially improving renal function and ameliorating hypertension after the removal of body lead (28). Our previous clinical trial also indicated that chelation therapy may slow the progression of renal insufficiency (11). Obviously, much remains to be explored in the mechanisms of lead-induced progression of renal insufficiency.
In conclusion, the findings of this study suggest that low-level environmental lead exposure is associated with accelerated deterioration of chronic renal insufficiency. The influence of environmental lead exposure on progression of renal insufficiency becomes more evident after long-term follow-up, in comparison with our previous observation. Even at levels far below the limits of the normal ranges of the general population, both increased BLL and BLB predict the accelerated progression of chronic renal diseases, although other treatable factors are under control. These results support continued efforts to reduce environmental lead exposure in patients with chronic renal diseases.
Acknowledgments
This study is supported in part by the academic grant from the National Science Council Foundation, Taiwan, Republic of China (91-2314-B-182A-129).
- © 2004 American Society of Nephrology
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