Utility of Spot Urine Specimens to Assess Tubular Secretion

Tubular Secretory Solute Clearance and HIV Infection

BACKGROUND

Tubular secretion is an important kidney function responsible for the clearance of numerous medications, including antibiotics and antivirals. It is unknown whether persons living with HIV have lower secretion compared with HIV-uninfected persons, which might predispose them to the risk of progressive kidney disease or adverse drug events.

SETTING AND METHODS

We evaluated a panel of 6 endogenous secretory solutes in 199 women living with HIV (WLWH) and 100 women without HIV enrolled in the Women's Interagency HIV Study. Secretory clearance was estimated as the urine-to-plasma ratio of each solute, with adjustment for urine tonicity. Using multivariable linear regression analysis, we compared differences in levels of secretory solute clearance between women with and without HIV and evaluated characteristics associated with secretion.

RESULTS

WLWH were older (median 40 vs. 38 years) but had similar estimated glomerular filtration rate (eGFR, 96 vs. 100 mL/minute/1.73 m 2 ) compared with those without HIV. African American and Latino race, diabetes, diastolic blood pressure, smoking, hepatitis C, peak HIV viral load, and current and nadir CD4 count were associated with differences in clearance of at least 1 marker after multivariable adjustment. The secretory clearance of 3 solutes (cinnamoylglycine, kynurenic acid, and pyridoxic acid) were on average 10%-15% lower among WLWH compared with those without HIV independent of eGFR, albuminuria and chronic kidney disease risk factors, including HCV, and injection drug use.

CONCLUSIONS

HIV is associated with reduced secretion among women with preserved eGFR. The implications of these findings for drug dosing and adverse events need to be evaluated.

Estimated Kidney Tubular Secretion and Kidney, Cardiovascular, and Mortality Outcomes in CKD: The Systolic Blood Pressure Intervention Trial

Rational & Objective

Many drugs, metabolites, and toxins are cleared by the kidneys via tubular secretion. Whether novel endogenous measures of tubular secretion provide information about kidney, cardiovascular, and mortality risk is uncertain.

Study Design

Longitudinal subgroup analysis of clinical trial participants.

Setting & Participants

2,089 Systolic Blood Pressure Intervention Trial participants with estimated glomerular filtration rate (eGFR) <60 mL/min/1.73 m² at baseline.

Exposure

Summary score incorporating urine-to-plasma ratios of 10 endogenous secretion markers measured in paired urine and plasma samples at baseline.

Outcome

The primary outcome was longitudinal change in eGFR. Secondary outcomes included chronic kidney disease (CKD) progression (≥50% eGFR decline or incident kidney failure requiring dialysis or kidney transplantation), a cardiovascular disease (CVD) composite (myocardial infarction, acute coronary syndrome, stroke, acute decompensated heart failure, or death from cardiovascular causes), and mortality.

Analytical Approach

Linear mixed-effect models were used to evaluate the association between the secretion score and change in eGFR, and Cox proportional hazards models were used to evaluate associations with CKD progression, CVD, and mortality.

Results

At baseline, mean age was 73 ± 9 years and eGFR was 46 ± 11 mL/min/1.73 m². During a median follow-up of 3.3 years, mean change in eGFR was -1.44% per year, and 72 CKD progression events, 272 CVD events, and 144 deaths occurred. In multivariable analyses, lower secretion score was associated with faster eGFR decline and greater risk of CKD progression, CVD, and mortality. After further adjustment for baseline eGFR and albuminuria, each 1-standard deviation lower secretion score was associated with faster eGFR decline (-0.65% per year; 95% CI, -0.84% to -0.46%), but not CKD progression (HR, 1.23; 95% CI, 0.96-1.58), CVD (HR, 1.02; 95% CI, 0.89-1.18), or mortality (HR, 0.90; 95% CI, 0.74-1.09). The secretion score association with eGFR decline appeared stronger in participants with baseline eGFR <45 mL/min/1.73 m² (P for interaction < 0.001).

Limitations

Persons with diabetes and proteinuria >1 g/d were excluded.

Conclusions

Among SPRINT participants with CKD, lower estimated tubular secretion was associated with faster eGFR decline, independent of baseline eGFR and albuminuria, but not with CKD progression, CVD, or mortality.

Markers of Kidney Tubular Secretion and Risk of Adverse Events in SPRINT Participants with CKD

BACKGROUND

Kidney tubular secretion is an essential mechanism for clearing many common antihypertensive drugs and other metabolites and toxins. It is unknown whether novel measures of tubular secretion are associated with adverse events (AEs) during hypertension treatment.

METHODS

Among 2089 SPRINT (Systolic Blood Pressure Intervention Trial) participants with baseline eGFR <60 ml/min per 1.73 m2, we created a summary secretion score by averaging across the standardized spot urine-to-plasma ratios of ten novel endogenous tubular secretion measures, with lower urine-to-plasma ratios reflecting worse tubular secretion. Multivariable Cox proportional hazards models were used to evaluate associations between the secretion score and risk of a composite of prespecified serious AEs (hypotension, syncope, bradycardia, AKI, electrolyte abnormalities, and injurious falls). The follow-up protocol for SPRINT routinely assessed two laboratory monitoring AEs (hyperkalemia and hypokalemia).

RESULTS

Overall, 30% of participants experienced at least one AE during a median follow-up of 3.0 years. In multivariable models adjusted for eGFR and albuminuria, lower (worse) secretion scores at baseline were associated with greater risk of the composite AE outcome (hazard ratio per 1-SD lower secretion score, 1.16; 95% confidence interval, 1.04 to 1.27). In analyses of the individual AEs, lower secretion score was associated with significantly greater risk of AKI, serious electrolyte abnormalities, and ambulatory hyperkalemia. Associations were similar across randomized treatment assignment groups.

CONCLUSION

Among SPRINT participants with CKD, worse tubular secretion was associated with greater risk of AEs, independent of eGFR and albuminuria.

Kidney Tubulointerstitial Fibrosis and Tubular Secretion

RATIONALE & OBJECTIVE

Tubular secretion plays an important role in the efficient elimination of endogenous solutes and medications, and lower secretory clearance is associated with risk of kidney function decline. We evaluated whether histopathologic quantification of interstitial fibrosis and tubular atrophy (IFTA) is associated with lower tubular secretory clearance in persons undergoing kidney biopsy.

STUDY DESIGN

Cross-sectional.

SETTINGS & PARTICIPANTS

The Boston Kidney Biopsy Cohort is a study of persons undergoing native kidney biopsies for clinical indications.

EXPOSURES

Semiquantitative score of IFTA reported by 2 trained pathologists.

OUTCOMES

We measured plasma and urine concentrations of 9 endogenous secretory solutes using a targeted liquid chromatography/mass spectrometry assay. We used linear regression to test associations of urine-to-plasma ratios (UPRs) of these solutes with IFTA score after controlling for estimated glomerular filtration rate (eGFR) and albuminuria.

RESULTS

Among 418 participants, mean age was 53 years, 51% were women, 64% were White, and 18% were Black. Mean eGFR was 50mL/min/1.73m2, and median urinary albumin-creatinine ratio was 819mg/g. Compared with individuals with≤25% IFTA, those with>50% IFTA had 12%-37% lower UPRs for all 9 secretory solutes. Adjusting for age, sex, race, eGFR, and urine albumin and creatinine levels attenuated the associations, yet a trend of lower secretion across groups remained statistically significant (P<0.05 for trend) for 7 of 9 solutes. A standardized composite secretory score incorporating UPR for all 9 secretory solutes using the min-max method showed similar results (P<0.05 for trend).

LIMITATIONS

Single time point and spot measures of secretory solutes.

CONCLUSIONS

Greater IFTA severity is associated with lower clearance of endogenous secretory solutes even after adjusting for eGFR and albuminuria.

Assessment of kidney proximal tubular secretion in critical illness

BACKGROUNDSerum creatinine concentrations (SCrs) are used to determine the presence and severity of acute kidney injury (AKI). SCr is primarily eliminated by glomerular filtration; however, most mechanisms of AKI in critical illness involve kidney proximal tubules, where tubular secretion occurs. Proximal tubular secretory clearance is not currently estimated in the intensive care unit (ICU). Our objective was to estimate the kidney clearance of secretory solutes in critically ill adults.METHODSWe collected matched blood and spot urine samples from 170 ICU patients and from a comparison group of 70 adults with normal kidney function. We measured 7 endogenously produced secretory solutes using liquid chromatography-tandem mass spectrometry. We computed a composite secretion score incorporating all 7 solutes and evaluated associations with 28-day major adverse kidney events (MAKE28), defined as doubling of SCr, dialysis dependence, or death.RESULTSThe urine-to-plasma ratios of 6 of 7 secretory solutes were lower in critically ill patients compared with healthy individuals after adjustment for SCr. The composite secretion score was moderately correlated with SCr and cystatin C (r = -0.51 and r = -0.53, respectively). Each SD higher composite secretion score was associated with a 25% lower risk of MAKE28 (95% CI 9% to 38% lower) independent of severity of illness, SCr, and tubular injury markers. Higher urine-to-plasma ratios of individual secretory solutes isovalerylglycine and tiglylglycine were associated with MAKE28 after accounting for multiple testing.CONCLUSIONAmong critically ill adults, tubular secretory clearance is associated with adverse outcomes, and its measurement could improve assessment of kidney function and dosing of essential ICU medications.FUNDINGGrants from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK/NIH) K23DK116967, the University of Washington Diabetes Research Center P30DK017047, an unrestricted gift to the Kidney Research Institute from the Northwest Kidney Centers, and the Vanderbilt O'Brien Kidney Center (NIDDK 5P30 DK114809-03). The funding sources had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; and preparation, review, or approval of the manuscript.

Impaired Tubular Secretion of Organic Solutes in Acute Kidney Injury

BACKGROUND

Impairment of kidney function is routinely assessed by measuring the accumulation of creatinine, an organic solute cleared largely by glomerular filtration. We tested whether the clearance of solutes that undergo tubular secretion is reduced in proportion to the clearance of creatinine in humans with AKI.

METHODS

Four endogenously produced organic solutes (phenylacetylglutamine [PAG], hippurate [HIPP], indoxyl sulfate [IS], and p-cresol sulfate [PCS]) were measured in spot urine and plasma samples from ten patients with AKI and 17 controls. Fractional clearance relative to creatinine was calculated to assess tubular secretion. Fractional clearance values were calculated in terms of the free, unbound levels of HIPP, IS, and PCS that bind to plasma proteins.

RESULTS

Fractional clearance values for PAG, HIPP, IS, and PCS were >1.0 in patients with AKI as well as controls, indicating that these solutes were still secreted by the tubules of the injured kidneys. Fractional clearance values were, however, significantly lower in patients with AKI than controls, indicating that kidney injury reduced tubular secretion more than glomerular filtration (AKI versus control: PAG, 2.1±0.7 versus 4.6±1.4, P<0.001; HIPP, 10±5 versus 15±7, P=0.02; IS, 10±6 versus 28±7, P<0.001; PCS, 3.3±1.8 versus 10±3, P<0.001). Free plasma levels rose out of proportion to total plasma levels for each of the bound solutes in AKI, so that calculating their fractional clearance in terms of their total plasma levels failed to reveal their impaired secretion.

CONCLUSIONS

Tubular secretion of organic solutes can be reduced out of proportion to glomerular filtration in AKI. Impaired secretion of protein-bound solutes may be more reliably detected when clearances are expressed in terms of their free, unbound levels in the plasma.

Tubular secretion of creatinine and kidney function: an observational study

BACKGROUND

Prior papers have been inconsistent regarding how much creatinine clearance (CrCl) overestimates glomerular filtration rate (GFR). A recent cross-sectional study suggested that measurement error alone could entirely account for the longstanding observation that CrCl/GFR ratio is larger when GFR is lower among patients with chronic kidney disease (CKD); but there have been no validation of this in other cohorts.

METHODS

To fill these gaps in knowledge regarding the relation between CrCl and GFR, we conducted cross-sectional and longitudinal analysis of the Modification of Diet in Renal Disease study (MDRD) and African American Study of Kidney Disease and Hypertension (AASK); and cross-sectional analysis of a clinical dataset from the Mayo Clinic of four different patient populations (CKD patients, kidney transplant recipients, post kidney donation subgroup and potential kidney donors). In the cross-sectional analyses (MDRD, AASK and Mayo Clinic cohort), we examined the relation between the CrCl/iothalamate GFR (iGFR) ratio at different categories of iGFR or different levels of CrCl. In the MDRD and AASK longitudinal analyses, we studied how the CrCl/iGFR ratio changed with those who had improvement in iGFR (CrCl) over time versus those who had worsening of iGFR (CrCl) over time.

RESULTS

Observed CrCl/iGFR ratios were generally on the lower end of the range reported in the literature for CKD (median 1.24 in MDRD, 1.13 in AASK and 1.25 in Mayo Clinic cohort). Among CKD patients in whom CrCl and iGFR were measured using different timed urine collections, CrCl/iGFR ratio were higher with lower iGFR categories but lower with lower CrCl categories. However, among CKD patients in whom CrCl and iGFR were measured using the same timed urine collections (which reduces dis-concordant measurement error), CrCl/iGFR ratio were higher with both lower iGFR categories and lower CrCl categories.

CONCLUSIONS

These data refute the recent suggestion that measurement error alone could entirely account for the longstanding observation that CrCl/GFR ratio increases as GFR decreases in CKD patients. They also highlight the lack of certainty in our knowledge with regard to how much CrCl actually overestimates GFR.

Comprehensive Assessment of Kidney Health in Acute Kidney Injury: Can It Be Achieved?

Acute kidney injury (AKI) is a frequent event in hospitalized patients, with an incidence that continues to rise, reaching as high as 70-80% in intensive care settings. The need for dialysis and progression to end-stage kidney disease (ESKD) after an episode of AKI is relatively low, from 5 to 20%. However, it is now recognized that patients with AKI may have very different kidney outcomes, varying from complete recovery, incipient chronic kidney disease (CKD), to progression to ESKD. Recent studies have shown that even mild AKI episodes can be associated with a 90% increased risk of developing CKD during long-term follow-up. There is a significant need to focus our efforts on factors that could mitigate the progression of kidney dysfunction and ultimately improve outcomes from AKI. The first step toward this goal encompasses a better understanding of tubular and glomerular alterations during and following an AKI episode. Our current approach, based solely on glomerular filtration rate (GFR), is flawed, since the loss of kidney function does not correspond to the degree of decline in estimated GFR (eGFR), and eGFR does not reflect tubular function. Changes in tubular concentration, reabsorptive and secretory capacity are recognized in AKI; however, they have not been incorporated in clinical assessments of overall kidney function. Here we review a few candidates to assess glomerular filtration/permeability, tubular dysfunction, and injury and how we expect these markers to alter during the development and recovery phase of AKI.