Efficacy and safety of sodium zirconium cyclosilicate in patients with baseline serum potassium level ≥ 5.5 mmol/L: pooled analysis from two phase 3 trials.

BACKGROUND: Reliable, timely-onset, oral treatments with an acceptable safety profile for patients with hyperkalemia are needed. We examined the efficacy and safety of sodium zirconium cyclosilicate (SZC; formerly ZS-9) treatment for ≤ 48 h in patients with baseline serum potassium level ≥ 5.5 mmol/L. METHODS: Data were pooled from two phase 3 studies (ZS-003 and HARMONIZE) among patients receiving SZC 10 g three times daily. Outcomes included mean and absolute change from baseline, median time to potassium level ≤ 5.5 and ≤ 5.0 mmol/L, and proportion achieving potassium level ≤ 5.5 and ≤ 5.0 mmol/L at 4, 24, and 48 h. Outcomes were stratified by baseline potassium. Safety outcomes were evaluated. RESULTS: At baseline, 125 of 170 patients (73.5%) had potassium level 5.5-< 6.0, 39 (22.9%) had potassium level 6.0-6.5, and 6 (3.5%) had potassium level > 6.5 mmol/L. Regardless of baseline potassium, mean potassium decreased at 1 h post-initial dose. By 4 and 48 h, 37.5% and 85.0% of patients achieved potassium level ≤ 5.0 mmol/L, respectively. Median (95% confidence interval) times to potassium level ≤ 5.5 and ≤ 5.0 mmol/L were 2.0 (1.1-2.0) and 21.6 (4.1-22.4) h, respectively. Fifteen patients (8.8%) experienced adverse events; none were serious. CONCLUSIONS: SZC 10 g three times daily achieved serum potassium reduction and normokalemia, with a favorable safety profile. TRIAL REGISTRATION: ClinicalTrials.gov identifiers: ZS-003: NCT01737697 and HARMONIZE: NCT02088073.

Preventability of Early Versus Late Hospital Readmissions in a National Cohort of General Medicine Patients.

Background: Many experts believe that hospitals with more frequent readmissions provide lower-quality care, but little is known about how the preventability of readmissions might change over the postdischarge time frame. Objective: To determine whether readmissions within 7 days of discharge differ from those between 8 and 30 days after discharge with respect to preventability. Design: Prospective cohort study. Setting: 10 academic medical centers in the United States. Patients: 822 adults readmitted to a general medicine service. Measurements: For each readmission, 2 site-specific physician adjudicators used a structured survey instrument to determine whether it was preventable and measured other characteristics. Results: Overall, 36.2% of early readmissions versus 23.0% of late readmissions were preventable (median risk difference, 13.0 percentage points [interquartile range, 5.5 to 26.4 percentage points]). Hospitals were identified as better locations for preventing early readmissions (47.2% vs. 25.5%; median risk difference, 22.8 percentage points [interquartile range, 17.9 to 31.8 percentage points]), whereas outpatient clinics (15.2% vs. 6.6%; median risk difference, 10.0 percentage points [interquartile range, 4.6 to 12.2 percentage points]) and home (19.4% vs. 14.0%; median risk difference, 5.6 percentage points [interquartile range, -6.1 to 17.1 percentage points]) were better for preventing late readmissions. Limitation: Physician adjudicators were not blinded to readmission timing, community hospitals were not included in the study, and readmissions to nonstudy hospitals were not included in the results. Conclusion: Early readmissions were more likely to be preventable and amenable to hospital-based interventions. Late readmissions were less likely to be preventable and were more amenable to ambulatory and home-based interventions. Primary Funding Source: Association of American Medical Colleges.

Effects of Sulfide Concentration and Dissolved Organic Matter Characteristics on the Structure of Nanocolloidal Metacinnabar.

Understanding the speciation of divalent mercury (Hg(II)) in aquatic systems containing dissolved organic matter (DOM) and sulfide is necessary to predict the conversion of Hg(II) to bioavailable methylmercury. We used X-ray absorption spectroscopy to characterize the structural order of mercury in Hg(II)-DOM-sulfide systems for a range of sulfide concentration (1-100 µM), DOM aromaticity (specific ultraviolet absorbance (SUVA254)), and Hg(II)-DOM and Hg(II)-DOM-sulfide equilibration times (4-142 h). In all systems, Hg(II) was present as structurally disordered nanocolloidal metacinnabar (ß-HgS). ß-HgS nanocolloids were significantly smaller or less ordered at lower sulfide concentration, as indicated by under-coordination of Hg(II) in ß-HgS. The size or structural order of ß-HgS nanocolloids increased with increasing sulfide abundance and decreased with increasing SUVA254 of the DOM. The Hg(II)-DOM or Hg(II)-DOM-sulfide equilibration times did not significantly influence the extent of structural order in nanocolloidal ß-HgS. Geochemical factors that control the structural order of nanocolloidal ß-HgS, which are expected to influence nanocolloid surface reactivity and solubility, should be considered in the context of mercury bioavailability.

Leucine-rich repeat kinase 2 modulates neuroinflammation and neurotoxicity in models of human immunodeficiency virus 1-associated neurocognitive disorders.

Leucine-rich repeat kinase 2 (LRRK2) is the single most common genetic cause of both familial and sporadic Parkinson's disease (PD), both of which share pathogenetic and neurologic similarities with human immunodeficiency virus 1 (HIV-1)-associated neurocognitive disorders (HAND). Pathologic LRRK2 activity may also contribute to neuroinflammation, because microglia lacking LRRK2 exposed to proinflammatory stimuli have attenuated responses. Because microglial activation is a hallmark of HIV-1 neuropathology, we have investigated the role of LRRK2 activation using in vitro and in vivo models of HAND. We hypothesize that LRRK2 is a key modulator of microglial inflammatory responses, which play a pathogenic role in both HAND and PD, and that these responses may cause or exacerbate neuronal damage in these diseases. The HIV-1 Tat protein is a potent neurotoxin produced during HAND that induces activation of primary microglia in culture and long-lasting neuroinflammation and neurotoxicity when injected into the CNS of mice. We found that LRRK2 inhibition attenuates Tat-induced pS935-LRRK2 expression, proinflammatory cytokine and chemokine expression, and phosphorylated p38 and Jun N-terminal kinase signaling in primary microglia. In our murine model, cortical Tat injection in LRRK2 knock-out (KO) mice results in significantly diminished neuronal damage, as assessed by microtubule-associated protein 2 (MAP2), class III ß-tubulin TUJ1, synapsin-1, VGluT, and cleaved caspase-3 immunostaining. Furthermore, Tat-injected LRRK2 KO animals have decreased infiltration of peripheral neutrophils, and the morphology of microglia from these mice were similar to that of vehicle-injected controls. We conclude that pathologic activation of LRRK2 regulates a significant component of the neuroinflammation associated with HAND.

Physician Perspectives on Factors Contributing to Readmissions and Potential Prevention Strategies: A Multicenter Survey.

BACKGROUND: The transition out of the hospital is a vulnerable time for patients, relying heavily on communication and coordination of resources across care settings. Understanding the perspectives of inpatient and outpatient physicians regarding factors contributing to readmission and potential preventive strategies is crucial in designing appropriately targeted readmission prevention efforts. OBJECTIVE: To examine and compare inpatient and outpatient physician opinions regarding reasons for readmission and interventions that might have prevented readmission. DESIGN: Cross-sectional multicenter study. PARTICIPANTS: We identified patients readmitted to general medicine services within 30 days of discharge at 12 US academic medical centers, and surveyed the primary care physician (PCP), discharging physician from the index admission, and admitting physician from the readmission regarding their endorsement of pre-specified factors contributing to the readmission and potential preventive strategies. MAIN MEASURES: We calculated kappa statistics to gauge agreement between physician dyads (PCP-discharging physician, PCP-admitting physician, and admitting-discharging physician). KEY RESULTS: We evaluated 993 readmission events, which generated responses from 356 PCPs (36 % of readmissions), 675 discharging physicians (68 % of readmissions), and 737 admitting physicians (74 % of readmissions). The most commonly endorsed contributing factors by both PCPs and inpatient physicians related to patient understanding and ability to self-manage. The most commonly endorsed preventive strategies involved providing patients with enhanced post-discharge instructions and/or support. Although PCPs and inpatient physicians endorsed contributing factors and potential preventive strategies with similar frequencies, agreement among the three physicians on the specific factors and/or strategies that applied to individual readmission events was poor (maximum kappa 0.30). CONCLUSIONS: Differing opinions among physicians on factors contributing to individual readmissions highlights the importance of communication between inpatient and outpatient providers at discharge to share their different perspectives, and suggests that multi-faceted, broadly applied interventions may be more successful than those that rely on individual providers choosing specific services based on perceived risk factors.

House Staff Participation in Patient Safety Reporting: Identification of Predominant Barriers and Implementation of a Pilot Program.

OBJECTIVES: Patient safety event (PSE) reporting is a critical element for healthcare organizations that are striving for continuous quality improvement. Although resident physicians routinely provide the majority of direct patient care, the level of their participation in PSE reporting historically has been low. In addition, as part of the Accreditation Council for Graduate Medical Education's Next Accreditation System, the Clinical Learning Environment Review site visit assesses residents' engagement in PSE reporting at each accredited academic institution. The objective of this study was to understand the common barriers to PSE reporting and design an intervention to increase the number of PSE reports by resident physicians. METHODS: We surveyed 304 residents and fellows to assess attitudes toward the PSE reporting system and identify barriers to submitting online PSE reports. Based on this analysis of barriers, we piloted interventions with the internal medicine residency program and measured their effect on resident PSE reporting. RESULTS: Of the survey respondents, 58% had never submitted a PSE report. The most commonly identified barriers were too much time required to submit a report (38% of all respondents), lack of education on how or what to report (37%), lack of feedback or change after reporting (19%), and concern for repercussions or lack of anonymity (13%). Based on this analysis of barriers, we piloted interventions with the internal medicine residency program to educate residents about PSE reporting through a reminder message in their orientation e-mail, informational slides at the end of conferences that described what and how to report, a pocket card with reporting instructions, and leadership encouragement during walk rounds by chief medical residents and the program director. Compared with the 10 weeks before the start of the intervention, the number of PSE reports submitted by internal medicine residents more than doubled, from 16 to 37 reports (P < 0.01). This increase in resident PSE reporting was sustained for 20 weeks despite the interventions lasting only 8 weeks. CONCLUSIONS: A resident-driven intervention that fostered a culture of encouragement for PSE reporting through leadership support and targeted education increased the number of PSE reports submitted by internal medicine residents at our health system. Hospitals and health systems should seek to understand the common barriers to PSE reporting from this important group of direct patient care providers and administer structured educational programs to encourage their participation.

Erratum to: Zn (II) and Cu (II) adsorption and retention onto iron oxyhydroxide nanoparticles: effects of particle aggregation and salinity.

[This corrects the article DOI: 10.1186/1467-4866-15-6.].

Zn(II) and Cu(II) adsorption and retention onto iron oxyhydroxide nanoparticles: effects of particle aggregation and salinity.

BACKGROUND: Iron oxyhydroxides are commonly found in natural aqueous systems as nanoscale particles, where they can act as effective sorbents for dissolved metals due to their natural surface reactivity, small size and high surface area. These properties make nanoscale iron oxyhydroxides a relevant option for the remediation of water supplies contaminated with dissolved metals. However, natural geochemical processes, such as changes in ionic strength, pH, and temperature, can cause these particles to aggregate, thus affecting their sorption capabilities and remediation potential. Other environmental parameters such as increasing salinity may also impact metal retention, e.g. when particles are transported from freshwater to seawater. RESULTS: After using synthetic iron oxyhydroxide nanoparticles and nanoparticle aggregates in batch Zn(II) adsorption experiments, the addition of increasing concentrations of chloride (from 0.1 M to 0.6 M) appears to initially reduce Zn(II) retention, likely due to the desorption of outer-sphere zinc surface complexes and subsequent formation of aqueous Zn-Cl complexes, before then promoting Zn(II) retention, possibly through the formation of ternary surface complexes (supported by EXAFS spectroscopy) which stabilize zinc on the surface of the nanoparticles/aggregates. In batch Cu(II) adsorption experiments, Cu(II) retention reaches a maximum at 0.4 M chloride. Copper-chloride surface complexes are not indicated by EXAFS spectroscopy, but there is an increase in the formation of stable aqueous copper-chloride complexes as chloride concentration rises (with CuCl(+) becoming dominant in solution at ~0.5 M chloride) that would potentially inhibit further sorption or encourage desorption. Instead, the presence of bidentate edge-sharing and monodentate corner-sharing complexes is supported by EXAFS spectroscopy. Increasing chloride concentration has more of an impact on zinc retention than the mechanism of nanoparticle aggregation, whereas aggregation condition is a stronger determinant of copper retention. CONCLUSIONS: Based on these model uptake/retention studies, iron oxyhydroxide nanoparticles show potential as a strategy to remediate zinc-contaminated waters that migrate towards the ocean. Copper retention, in contrast, appears to be optimized at an intermediate salinity consistent with brackish water, and therefore may release considerable fractions of retained copper at higher (e.g. seawater) salinity levels.

Fluvial transport and surface enrichment of arsenic in semi-arid mining regions: examples from the Mojave Desert, California.

As a result of extensive gold and silver mining in the Mojave Desert, southern California, mine wastes and tailings containing highly elevated arsenic (As) concentrations remain exposed at a number of former mining sites. Decades of weathering and erosion have contributed to the mobilization of As-enriched tailings, which now contaminate surrounding communities. Fluvial transport plays an intermittent yet important and relatively undocumented role in the migration and dispersal of As-contaminated mine wastes in semi-arid climates. Assessing the contribution of fluvial systems to tailings mobilization is critical in order to assess the distribution and long-term exposure potential of tailings in a mining-impacted environment. Extensive sampling, chemical analysis, and geospatial mapping of dry streambed (wash) sediments, tailings piles, alluvial fans, and rainwater runoff at multiple mine sites have aided the development of a conceptual model to explain the fluvial migration of mine wastes in semi-arid climates. Intense and episodic precipitation events mobilize mine wastes downstream and downslope as a series of discrete pulses, causing dispersion both down and lateral to washes with exponential decay behavior as distance from the source increases. Accordingly a quantitative model of arsenic concentrations in wash sediments, represented as a series of overlapping exponential power-law decay curves, results in the acceptable reproducibility of observed arsenic concentration patterns. Such a model can be transferable to other abandoned mine lands as a predictive tool for monitoring the fate and transport of arsenic and related contaminants in similar settings. Effective remediation of contaminated mine wastes in a semi-arid environment requires addressing concurrent changes in the amounts of potential tailings released through fluvial processes and the transport capacity of a wash.

Formation of nanocolloidal metacinnabar in mercury-DOM-sulfide systems.

Direct determination of mercury (Hg) speciation in sulfide-containing environments is confounded by low mercury concentrations and poor analytical sensitivity. Here we report the results of experiments designed to assess mercury speciation at environmentally relevant ratios of mercury to dissolved organic matter (DOM) (i.e., <4 nmol Hg (mg DOM)(-1)) by combining solid phase extraction using C(18) resin with extended X-ray absorption fine structure (EXAFS) spectroscopy. Aqueous Hg(II) and a DOM isolate were equilibrated in the presence and absence of 100 µM total sulfide. In the absence of sulfide, mercury adsorption to the resin increased as the Hg:DOM ratio decreased and as the strength of Hg-DOM binding increased. EXAFS analysis indicated that in the absence of sulfide, mercury bonds with an average of 2.4 ± 0.2 sulfur atoms with a bond length typical of mercury-organic thiol ligands (2.35 Å). In the presence of sulfide, mercury showed greater affinity for the C(18) resin, and its chromatographic behavior was independent of Hg:DOM ratio. EXAFS analysis showed mercury-sulfur bonds with a longer interatomic distance (2.51-2.53 Å) similar to the mercury-sulfur bond distance in metacinnabar (2.53 Å) regardless of the Hg:DOM ratio. For all samples containing sulfide, the sulfur coordination number was below the ideal four-coordinate structure of metacinnabar. At a low Hg:DOM ratio where strong binding DOM sites may control mercury speciation (1.9 nmol mg(-1)) mercury was coordinated by 2.3 ± 0.2 sulfur atoms, and the coordination number rose with increasing Hg:DOM ratio. The less-than-ideal coordination numbers indicate metacinnabar-like species on the nanometer scale, and the positive correlation between Hg:DOM ratio and sulfur coordination number suggests progressively increasing particle size or crystalline order with increasing abundance of mercury with respect to DOM. In DOM-containing sulfidic systems nanocolloidal metacinnabar-like species may form, and these species need to be considered when addressing mercury biogeochemistry.

New technique for quantification of elemental Hg in mine wastes and its implications for mercury evasion into the atmosphere.

Mercury in the environment is of prime concern to both ecosystem and human health. Determination of the molecular-level speciation of Hg in soils and mine wastes is important for understanding its sequestration, mobility, and availability for methylation. Extended X-ray absorption fine structure (EXAFS) spectroscopy carried out under ambient P-T conditions has been used in a number of past studies to determine Hg speciation in complex mine wastes and associated soils. However, this approach cannot detect elemental (liquid) mercury in Hg-polluted soils and sediments due to the significant structural disorder of liquid Hg at ambient-temperature. A new sample preparation protocol involving slow cooling through the crystallization temperature of Hg(0) (234 K) results in its transformation to crystalline α-Hg(0). The presence and proportion of Hg(0), relative to other crystalline Hg-bearing phases, in samples prepared in this way can be quantified by low-temperature (77 K) EXAFS spectroscopy. Using this approach, we have determined the relative concentrations of liquid Hg(0) in Hg mine wastes from several sites in the California Coast Range and have found that they correlate well with measured fluxes of gaseous Hg released during light and dark exposure of the same samples, with higher evasion ratios from samples containing higher concentrations of liquid Hg(0). Two different linear relationships are observed in plots of the ratio of Hg emission under light and dark conditions vs % Hg(0), corresponding to silica-carbonate- and hot springs-type Hg deposits, with the hot springs-type samples exhibiting higher evasion fluxes than silica-carbonate type samples at similar Hg(0) concentrations. Our findings help explain significant differences in Hg evasion data for different mine sites in the California Coast Range.

Impacts of Post-Hospitalization Accessible Health Technology and Caregiver Support on 90-Day Acute Care Use and Self-Care Assistance: A Randomized Clinical Trial.

Hospitalized patients often are readmitted soon after discharge, with many hospitalizations being potentially preventable. The authors evaluated a mobile health intervention designed to improve post-hospitalization support for older adults with common chronic conditions. All participants enrolled with an informal caregiver or "CarePartner" (CP). Intervention patients received automated assessment and behavior change calls. CPs received automated, structured feedback following each assessment. Clinicians received alerts about serious problems identified during patient calls. Controls had a 65% greater risk of hospitalization within 90 days post discharge than intervention patients (P = .041). For every 6.8 enrollees, the intervention prevented 1 rehospitalization or emergency department encounter. The intervention improved physical functioning at 90 days (P = .012). The intervention also improved medication adherence and indicators of the quality of communication with CPs (all P < .01). Automated telephone patient monitoring and self-care advice with feedback to primary care teams and CPs reduces readmission rates over 90 days.

Bracing for the Storm: One Health Care System's Planning for the COVID-19 Surge.

PROBLEM: University of Washington Medicine (UW Medicine), an academic health system in Washington State, was at the epicenter of the first outbreak of the COVID-19 pandemic in the United States. The extent of emergency activation needed to adequately respond to this global pandemic was not immediately known, as the evolving situation differed significantly from any past disaster response preparations in that there was potential for exponential growth of infection, unproven mitigation strategies, serious risk to health care workers, and inadequate supply chains for critical equipment. APPROACH: The rapid transition of the UW Medicine system to account for projected COVID-19 and usual patient care, while balancing patient and staff safety and conservation of resources, represents an example of an adaptive disaster response. KEY INSIGHTS: Although our organization's ability to meet the needs of the public was uncertain, we planned and implemented changes to space, supply management, and staffing plans to meet the influx of patients across our clinical entities. The surge management plan called for specific actions to be implemented based on the level of activity. This article describes the approach taken by UW Medicine as we braced for the storm.

One Academic Health System's Early (and Ongoing) Experience Responding to COVID-19: Recommendations From the Initial Epicenter of the Pandemic in the United States.

On January 19, 2020, the first case of a patient with coronavirus disease 2019 (COVID-19) in the United States was reported in Washington State. On February 29, 2020, a patient infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) passed away in a hospital in Seattle-King County, the first reported COVID-19-related death in the United States. That same day, a skilled nursing and rehabilitation facility in the county reported that several of its residents tested positive for SARS-CoV-2 and that many staff had symptoms compatible with COVID-19.The University of Washington Medicine health system (UW Medicine), which is based in Seattle-King County and provides quaternary care for the region, was one of several health care organizations called upon to address this growing crisis. What ensued was a series of swiftly enacted decisions and activities at UW Medicine, in partnership with local, state, and national public health agencies, to respond to the COVID-19 pandemic. Tapping into the multipronged mission areas of academic medicine, UW Medicine worked to support the community, innovate in science and clinical practice; lead policy and practice guideline development; and adopt changes as the crisis unfolded. In doing so, health system leaders had to balance their commitments to students, residents and fellows, researchers, faculty, staff, and hospital and health center entities, while ensuring that patients continued to receive cutting-edge, high-quality, safe care. In this Invited Commentary, the authors highlight the work and challenges UW Medicine has faced in responding to the global COVID-19 pandemic.

Effects of Accessible Health Technology and Caregiver Support Posthospitalization on 30-Day Readmission Risk: A Randomized Trial.

INTRODUCTION: Patients with chronic illness often require ongoing support postdischarge. This study evaluated a simple-to-use, mobile health-based program designed to improve postdischarge follow-up via (1) tailored communication to patients using automated calls, (2) structured feedback to informal caregivers, and (3) automated alerts to clinicians about urgent problems. METHODS: A total of 283 patients with common medical diagnoses, including chronic obstructive pulmonary disease, coronary artery disease, pneumonia, and diabetes, were recruited from a university hospital, a community hospital, and a US Department of Veterans Affairs hospital. All patients identified an informal caregiver or "care partner" (CP) to participate in their postdischarge support. Patient-CP dyads were randomized to the intervention or usual care. Intervention patients received weekly automated assessment and behavior change calls. CPs received structured e-mail feedback. Outpatient clinicians received fax alerts about serious problems. Primary outcomes were 30-day readmission rate and the combined outcome of readmission/emergency department (ED) use. Information about postdischarge outpatient visits, rehospitalizations, and ED encounters was obtained from medical records. RESULTS: Overall, 11.4% of intervention patients and 17.9% of controls were rehospitalized within 30 days postdischarge (hazard ratio [HR]: 0.59; 95% confidence interval [CI]: 0.31-1.11; p = 0.102). Compared to intervention patients with other illnesses, those with pulmonary diagnoses generated the most clinical alerts (p = 0.004). Pulmonary patients in the intervention group showed significantly reduced 30-day risk of rehospitalization relative to controls (HR: 0.31; 95% CI: 0.11-0.87; p = 0.026). CONCLUSION: The CP intervention did not improve 30-day readmission rates overall, although post hoc analyses suggested that it may be promising among patients with pulmonary diagnoses.

Implementation of lean thinking: one health system's journey.

BACKGROUND: Lean Thinking is a management philosophy derived from the manufacturing industry, where Toyota has long been the gold standard. Health care organizations have started to apply this approach to patient care. After initial experimentation, the University of Michigan Health System (UMHS) has adopted Lean Thinking as its uniform approach to quality improvement and is striving to become a complete Lean organization. PROJECTS: In 2005, the senior leadership selected an initial set of projects in areas that traced the patient's journey across different care settings within our health system. Four of the projects were as follows: orthopedic surgery clinic scheduling, radiation oncology therapy, peripherally inserted central catheter (PICC) services, and coordination of care to the outpatient setting. LESSONS FROM LEAN THINKING: Lean Thinking encourages service providers to focus on value as defined by the customer and the relentless elimination of waste that impedes the flow of value. A series of learning projects were conducted to test whether Lean methods would work at UMHS. The following factors were found to be key to LEAN PROJECT SUCCESS: expert guidance for initial efforts, leadership in the form of clinical champions and senior management support of the improvement work, frontline worker engagement in mapping out "current state" processes, identifying waste and designing an improved "future state," using metrics to develop and track interventions, and defining realistic project scope. FINAL REFLECTIONS: As UMHS's experience applying Lean Thinking to our patient care processes has grown, so have support, enthusiasm, and expertise within the organization. UMHS's Lean Thinking system, now known as the Michigan Quality System, has emerged as the core improvement strategy.

Stable cluster formation in aqueous suspensions of iron oxyhydroxide nanoparticles.

Metal oxide and oxyhydroxide nanoparticles are important components of natural aqueous systems and have application in photocatalysis. Uncoated (oxyhydr)oxide nanoparticles can form charge-stabilized colloids in water, but the precise regimes of dispersion and aggregation have been determined for very few nanomaterials. We studied the colloidal behavior of approximately 6 nm nanoparticles of iron oxyhydroxide (FeOOH), a common natural nanoscale colloid, and found that these nanoparticles formed stable suspended clusters under a range of aqueous conditions. Light and X-ray scattering methods show that suspended fractal nanoclusters are formed between pH 5 and 6.6 with well-defined maximum diameters that can be varied from 25 nm to approximately 1000 nm. The nanoclusters retain a very high surface area, and persist in suspension for at least 10 weeks in solution. The process is partially reversible because optically transparent suspensions are regained when nanoparticles that aggregated and settled at pH >7 are adjusted to pH 4 without stirring. However, completely redispersed nanoparticles are not obtained even after one month. Because nanocluster formation is controlled predominantly by surface charge, we anticipate that many metal oxide and other inorganic nanoparticles will exhibit equivalent cluster-forming behavior. Our results indicate that natural nanoparticles could form stable nanoclusters in groundwater that are likely to be highly mobile, with implications for the long-range transport of surface sorbed contaminants.