Per Sjöström

Determination of the production rate and non-renal clearance of cystatin C and estimation of the glomerular filtration rate from the serum concentration of cystatin C in humans.

Cystatin C has been proposed as an endogenous marker for measuring glomerular filtration rate (GFR) and is regarded as being equivalent to or better than creatinine. However, there are no published data on the production rate (Cyspr) or on the non‐renal clearance of cystatin C (CLnr) in humans, which are essential parameters for GFR calculation. GFR was determined by measuring the plasma clearance of iohexol. Cystatin C, creatinine, urea and albumin were determined on the same serum samples as iohexol; 381 patients with a GFR range of 12–151 ml/min/1.73 m2, and 70 patients on haemodialysis were evaluated. Renal clearance of cystatin C (CLr) equals GFR * S (the sieving coefficient). Plasma clearance (CL) = CLr+CLnr. The relationship between Cyspr and the elimination rate (CL * serum‐cystatin C) can be expressed as Cyspr = (S * GFR+CLnr) * serum‐cystatin C. Assuming that the unknown values of Cyspr and CLnr are independent of GFR, the equation can be solved from GFR (iohexol clearance) and serum cystatin C (s‐Cys) patient data. For S = 1, we found Cyspr = 0.124±0.023 mg/min/1.73 m2 and Clnr = 22.3 ml/min/1.73 m2. For S = 0.94, found in rats, the values will be Cyspr = 0.117 mg/min/1.73 m2 and Clnr = 21 ml/min/1.73 m2 and S‐Cys in 70 patients on chronic haemodialysis was found to be 5.74±1.15 mg/l, in agreement with a calculated value of 5.56 mg/l (s‐Cys = 124/22.3) for GFR = zero. The mean value of the calculated Clnr for the 70 patients was 22.7±6.6 ml/min/1.73 m2, which confirms the calculated level and indicates its biological variation. We thus propose the following formula for calculating GFR using the values found for CLnr and Cyspr in this study: GFR = 124/s‐Cys−22.3 ml/min/1.73 m2, where serum cystatin C concentration is given as mg/l.

Generation of a New Cystatin C–Based Estimating Equation for Glomerular Filtration Rate by Use of 7 Assays Standardized to the International Calibrator

BACKGROUND Many different cystatin C-based equations exist for estimating glomerular filtration rate. Major reasons for this are the previous lack of an international cystatin C calibrator and the nonequivalence of results from different cystatin C assays. METHODS Use of the recently introduced certified reference material, ERM-DA471/IFCC, and further work to achieve high agreement and equivalence of 7 commercially available cystatin C assays allowed a substantial decrease of the CV of the assays, as defined by their performance in an external quality assessment for clinical laboratory investigations. By use of 2 of these assays and a population of 4690 subjects, with large subpopulations of children and Asian and Caucasian adults, with their GFR determined by either renal or plasma inulin clearance or plasma iohexol clearance, we attempted to produce a virtually assay-independent simple cystatin C-based equation for estimation of GFR. RESULTS We developed a simple cystatin C-based equation for estimation of GFR comprising only 2 variables, cystatin C concentration and age. No terms for race and sex are required for optimal diagnostic performance. The equation, [Formula: see text] is also biologically oriented, with 1 term for the theoretical renal clearance of small molecules and 1 constant for extrarenal clearance of cystatin C. CONCLUSIONS A virtually assay-independent simple cystatin C-based and biologically oriented equation for estimation of GFR, without terms for sex and race, was produced.

Extensive tubular secretion and reabsorption of creatinine in humans.

The validity of creatinine as a marker for the glomerular filtration rate was studied in 8 healthy volunteers in different stages of hydration and during large variations in urinary flow rates. The urine flow was 8.4 ml/min in a dehydrated state (due to furosemide infusion; 8 mg/h) and raised to 23.2 ml/min after rapid rehydration. The creatinine to inulin clearance ratio changed considerably from 1.47 in rehydrated state, indicating a substantial tubular secretion of creatinine, to 1.05 in dehydrated state, indicating a reabsorption of creatinine almost equal to secretion. Thus, substantial tubular secretion and reabsorption of creatinine, changing in relative importance in relation to the degree of hydration, make creatinine clearance an unreliable marker for the glomerular filtration rate.

Delayed tolerance to furosemide diuresis. Influence of angiotensin converting enzyme inhibition by lisinopril.

The role of the renin-angiotensin-aldosterone system in the development of tolerance to the diuretic effect of furosemide was investigated in 12 healthy male volunteers. Furosemide in a dose of 40 mg daily for one week had a brisk acute diuretic effect, but did not lead to dehydration, hyponatremia or fall in blood pressure. The reason for this was a reduction in sodium excretion between doses (rebound effect) and a decrease in sensitivity to furosemide from day 1 to day 7. The latter phenomenon is referred to as delayed tolerance to furosemide. Inhibition of angiotensin converting enzyme with lisinopril 20 mg daily did not change the renal furosemide excretion rate, the renal sensitivity to furosemide or the tolerance development. Thus, delayed tolerance to furosemide diuresis was not related to dehydration or activation of the renin-angiotensin-aldosterone system. Other mechanisms, probably intrarenal, will have to be looked for.

Validation of the Lund-Malmö, Chronic Kidney Disease Epidemiology (CKD-EPI) and Modification of Diet in Renal Disease (MDRD) equations to estimate glomerular filtration rate in a large Swedish clinical population.

Abstract Objective. The aim of this study was to validate externally the Swedish Lund–Malmö revised creatinine-based glomerular filtration rate (GFR) equations (LM Revised) in a Swedish cohort in comparison with the North American Modification of Diet in Renal Disease (MDRD) and Chronic Kidney Disease Epidemiology (CKD-EPI) equations. Material and methods. The study included 1397 examinations [median age 61 years, median body mass index (BMI) 26 kg/m2] in 996 patients referred for iohexol clearance (median 44 ml/min/1.73 m2). Bias, precision [interquartile range (IQR)], accuracy expressed as percentage of estimates ± 10% (P 10) and ± 30% (P 30) of measured GFR, and classification ability for five GFR stages (<15, 15–29, 30–59, 60–89 and ≥90 ml/min/1.73 m2) were compared. Results. Overall, all three equations performed satisfactorily: LM Revised, MDRD, CKD-EPI showed, respectively, a median bias of –5.8%, –2.2% and 1.7%, IQR 11.9, 12.3 and 11.7 ml/min/1.73 m2, P 10 35%, 34% and 38%, P 30 84%, 79% and 79% and correctly classified GFR stages 68%, 65% and 69%. LM Revised was at least as accurate in terms of P 30 as the other equations at GFR intervals <90, while CKD-EPI was the only unbiased and the most accurate equation at ≥90 ml/min/1.73 m2. LM Revised was more stable in terms of bias and accuracy across age and BMI groups than MDRD and CKD-EPI. Both MDRD and CKD-EPI overestimated measured GFR among elderly patients and in the small group of underweight men. Conclusion. The ideal all-purpose GFR prediction equation does not exist. LM Revised should be preferred in patients with suspected or known renal insufficiency, while CKD-EPI is most useful in settings where patients with no a priori suspicion of renal impairment are evaluated. Differences in creatinine measurements between laboratories may limit the generalizability of the present validation.

On the Mechanism of Acute Tolerance to Furosemide Diuresis

The renal response to continuous furosemide infusion (8 mg/h) and subsequent ECV changes was studied in 8 healthy volunteers. Furosemide increased urine flow from a basal flow of 4.3 ml/min to a maximum of 15.4 ml/min. During dehydration (-1.8 kg) the diuresis decreased to 8.4 ml/min. The sodium, chloride and potassium excretion likewise decreased. This reduction in diuretic effect (acute tolerance) was accompanied by significantly increased plasma levels of norepinephrine (1.38 to 2.14 nmol/l), PRA (0.52 to 1.13 ng/ml/h) and aldosterone (0.29 to 0.45 nmol/l). After rehydration the urine flow increased to 23.1 ml/min. The changes in diuretic response from initial effect to the dehydrated state and after rehydration were mainly a consequence of changed renal sensitivity to furosemide (urinary excretion of 21 to 14 to 35 mumol Na+ per microgram furosemide excreted). It is proposed that activation of the sympathetic nervous system and/or the renin-angiotensin-aldosterone system may play a role in mediating the acute tolerance to furosemide diuresis. The relative importance of each remains to be clarified.

Cystatin C as a filtration marker – haemodialysis patients expose its strengths and limitations

Objective. The reliability of serum cystatin C (s‐Cys) as a filtration marker depends on the intra‐ and inter‐individual variation and influence of non‐renal factors of its production rate (Cyspr), non‐renal clearance (CLnr) and sieving coefficient (S). Haemodialysis patients with no residual renal function would be the best population in which to investigate these variables, which otherwise require reliable GFR measurements. Material and methods. Seventy‐nine haemodialysis (HD) patients with negligible residual renal function (Group 1) were investigated and compared with 55 HD patients with varying degrees of residual renal function (Group 2) and 923 non‐dialysis patients (Group 3). The equation eGFR = Cyspr/s‐Cys−CLnr was used to analyse the turnover and variation of cystatin C. Results. A formula for estimating GFR, eGFR = 99/s‐Cys−14.1, calculated from Group 3, was shown to fit the HD patients. The measured s‐Cys in Group 1 was 6.9±0.9 and 6.4±1.1 mg/L in Group 2. The calculated 95 % confidence interval of eGFR of ±(30–40) % increased sharply below GFR 20 mL/min/1.73 m2, which means that s‐Cys cannot be used for calculating low GFR, including the residual GFR of dialysis patients. Conclusions. Nicotine users in Group 1 had significantly higher s‐Cys than non‐users (7.5±0.9 mg/L compared to 6.7±0.8; p = 0.0008), which may be a factor to include in the eGFR formulae. However, s‐Cys was independent of non‐renal factors such as sex, age, LBM, body weight, malnutrition and CRP and also of changes in CRP.

Acute Tolerance to Furosemide. Pretreatment with Captopril or Prazosin does not Influence Diuresis and Natriuresis

To investigate whether the development of acute tolerance to furosemide in human subjects could be prevented or delayed by angiotensin converting enzyme inhibition or alpha 1-receptor blockade, a study was conducted on healthy volunteers. The protocol on the experimental days was identical except for pre-treatment with placebo, captopril or prazosin. During continuous furosemide infusion with urinary furosemide excretion at a constant rate, the subjects became progressively dehydrated, with a maximal decrease in plasma volume of 9-11%. The diuretic/natriuretic response to furosemide was similar in the three protocols. Acute tolerance to diuresis developed earlier than that to natriuresis, again with no differences between the protocols. Not until the plasma volume had decreased by 9% did the natriuresis diminish significantly. In the placebo and captopril protocols acute tolerance was caused mainly by a decreased glomerular filtration rate, and in the prazosin protocol mainly by increased tubular reabsorption. It is concluded that neither ACE inhibition nor alpha 1-receptor blockade prevented or delayed the acute tolerance to furosemide. The results suggest that acute tolerance to furosemide can be induced through different but complementary homeostatic mechanisms in the kidney.

GFR estimation based on standardized creatinine and cystatin C : A European multicenter analysis in older adults

Abstract Background: Although recommended by the Kidney Disease Improving Global Outcomes, the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPICR) creatinine equation was not targeted to estimate glomerular filtration rate (eGFR) among older adults. The Berlin Initiative Study (BIS1CR) equation was specifically developed in older adults, and the Lund-Malmö revised (LMRCR) and the Full Age Spectrum (FASCR) equations have shown promising results in older adults. Our aim was to validate these four creatinine equations, including addition of cystatin C in a large multicenter cohort of Europeans ≥70 years. Methods: A total of 3226 individuals (2638 with cystatin C) underwent GFR measurement (mGFR; median, 44 mL/min/1.73 m2) using plasma iohexol clearance. Bias, precision (interquartile range [IQR]), accuracy (percent of estimates ±30% of mGFR, P30), eGFR accuracy diagrams and probability diagrams to classify mGFR<45 mL/min/1.73 m2 were compared. Results: The overall results of BIS1CR/CKD-EPICR/FASCR/LMRCR were as follows: median bias, 1.7/3.6/0.6/−0.7 mL/min/1.73 m2; IQR, 11.6/12.3/11.1/10.5 mL/min/1.73 m2; and P30, 77.5%/76.4%/80.9%/83.5% (significantly higher for LMR, p<0.001). Substandard P30 (<75%) was noted for all equations at mGFR<30 mL/min/1.73 m2, and at body mass index values <20 and ≥35 kg/m2. LMRCR had the most stable performance across mGFR subgroups. Only LMRCR and FASCR had a relatively constant small bias across eGFR levels. Probability diagrams exhibited wide eGFR intervals for all equations where mGFR<45 could not be confidently ruled in or out. Adding cystatin C improved P30 accuracy to 85.7/86.8/85.7/88.7 for BIS2CR+CYS/CKD-EPICR+CYS/FASCR+CYS/MEANLMR+CAPA. Conclusions: LMRCR and FASCR seem to be attractive alternatives to CKD-EPICR in estimating GFR by creatinine-based equations in older Europeans. Addition of cystatin C leads to important improvement in estimation performance.

PHARMACOKINETIC-PHARMACODYNAMIC RELATIONSHIP OF PIRETANIDE IN HEALTHY AND UREMIC SUBJECTS Determinants of the Diuretic Effect of a Loop Diuretic

The pharmacokinetics of the loop diuretic piretanide and its diuretic effects were studied in 6 healthy volunteers, 12 pre-dialysis (GFR 7-28 ml/min) and 10 dialysis patients (c-creat. 1-7 ml/min). Single doses up to 96 mg i.v. and orally were well tolerated and audiometry showed no hearing changes. Pharmacokinetic data showed rapid and almost complete absorption (bioavailability 92%) and a rapid elimination with renal clearance of 50% of the total 200 ml/min in the normals and renal clearance of about 50% of actual GFR in the patients. Extrarenal clearance was the same in normals and patients. The rapid extrarenal elimination reduces the risk of accumulation in renal patients but also reduces the active fraction of the dosage being cleared by the kidneys. Therefore, a high dosage and high plasma levels of piretanide were necessary for diuretic effect in uremic patients. The relation between the urinary piretanide excretion rate and the chloruretic effect was similar in normals and uremic patients; Cl- excretion increased 40 mMol per mg piretanide excreted.