Charna Albert
October 2024—In patients at risk for chronic kidney disease, clinical practice guidelines released this year recommend the combined creatinine and cystatin C-based estimated glomerular filtration rate over the creatinine-based eGFR when cystatin C is available.
“Cystatin C isn’t perfect itself, but a combination of the two seems to hit the sweet spot,” Angela Ferguson, PhD, D(ABCC), said in a session at the ADLM meeting in July.
Dr. Ferguson is associate professor at the University of Missouri-Kansas City School of Medicine and co-director of clinical chemistry, director of immunology, and director of point-of-care testing in the Department of Pathology and Laboratory Medicine, Children’s Mercy Kansas City. The guidelines were released by the Kidney Disease: Improving Global Outcomes chronic kidney disease work group (Stevens PE, et al. Kidney Int. 2024;105[4S]:S117–S314).
When it comes to pediatric patients, said Darcy Weidemann, MD, MHS, Dr. Ferguson’s co-presenter and a pediatric nephrologist at Children’s Mercy, adding cystatin C to the estimating equations may lead to more precise estimates of GFR.
“Creatinine is a particularly poor marker of kidney function in patients with low muscle mass,” Dr. Weidemann said. “This is especially relevant in pediatrics, where use of creatinine alone can be misleading. Due to the unique developmental and growth considerations of a growing child, the ‘normal’ reference ranges are much wider in laboratory reference ranges, depending on how much they stratify out normal by age, and may not adequately cover the true spectrum of normal GFRs for a child of a particular age and size.” For example, she said, if a normal creatinine for an infant should be around 0.2, a value of 0.6 has a high risk of being missed as abnormal, “although that would be considered fairly significant impaired kidney function for a three-month-old baby.”
Cystatin C is particularly helpful at the higher GFR ranges, “where under-recognition of impaired kidney function is more likely using creatinine alone,” Dr. Weidemann said. “So that’s where a lot of the excitement about adding in cystatin C is, particularly in our pediatrics world—to improve that diagnosis.”
Dr. Ferguson in her talk reviewed three of the contemporary pediatric GFR estimating equations and described how she and her colleagues at Children’s Mercy implemented eGFR directly in the laboratory information system. Previously, the process required that the clinicians take values from the patient chart, plug them into online eGFR calculators, and transpose the information back into the electronic health record. The eGFR values were then saved in the notes section of the chart, apart from the other laboratory data. The nephrologists made a request of the laboratory: “Can you report the eGFR in the chart?” Which equation to use became the question for the laboratory.
Dr. Ferguson discussed the following three equations:
- Creatinine-based “bedside” CKiD equation (2009): eGFR = 41.3 × (height/SCr)
- Cystatin C-based equation (2012): eGFR = 70.69 × (cysC)-0.931
- Creatinine-cystatin C-based CKiD equation (2012): eGFR = 39.8 × [ht/SCr]0.456 × [1.8/cysC]0.418 × [30/BUN]0.079 × [1.076male] [1.00female] × [ht/1.4]0.179
A Children’s Mercy nephrologist was an investigator on a study that aimed to revisit the CKiD bedside equation (Pierce CB, et al. Kidney Int. 2021;99[4]:948–956). “This paper was in press at the time,” Dr. Ferguson said, “and we wanted to focus on the equations they were developing.” Using data from the Chronic Kidney Disease in Children Study, Pierce, et al., developed and validated four new eGFR equations for clinical use in children and young adults. The equations are referred to as the CKiD under 25 equations, or U25.
According to Pierce, et al., the data used to develop the original CKiD bedside equation came primarily from children aged eight to 15 and lacked an adequate sample of both young children under five and young adults 18 and over. “The purpose of their study was to include more patients and broaden the age range,” Dr. Ferguson said. Other study aims were to assess if the constant (K=41.3) for height/SCr should be modified by sex and age to yield less biased estimates of measured GFR—most notably in children under five and adults 18 to 25—and to develop a complementary estimating equation based on cystatin C, as well as compare the new equations with previously published eGFR equations for children and young adults.
The authors used 2,655 observations from 928 participants. For each marker, one equation used a sex-dependent K; in the other, K was sex- and age-dependent. GFR was measured directly by plasma iohexol disappearance. Use of an age-dependent K with ht/SCr models reduced average bias, notably in young children and young adults; age-dependent cystatin C models produced similar agreement to using a constant K in children under 18 years, but reduced bias in young adults. “They were able to improve the estimation of GFR, particularly in young children and young adults,” Dr. Ferguson said, as well as offer a single equation for use across the full pediatric age spectrum. “At our institution we take patients up to age 22,” she noted.
In Fig. 1 are the CKiD under 25 equations.
One caveat: “These equations were developed and validated in patients with mild to moderate CKD. It’s not recommended to use them in general screening because we don’t know the validity of these equations in the healthy patient,” Dr. Ferguson said.
“But the equations perform quite well,” she continued. “There’s an average bias in all age and sex subgroups within 10 percent of the measured GFR.” The age-dependent proposed equations, Pierce, et al., write, were evaluated alongside estimated GFRs from 11 other published equations for pediatrics and young adults. They add, “Only our proposed equations yielded non-significant bias and within 30% accuracy values greater than 85% in both the pediatric and young adult subpopulations.”
“One other point was made,” Dr. Ferguson said. “If you take eGFR-creatinine and eGFR-cystatin and average them, that gives you the best, most accurate estimation of GFR.” When the two estimates are available, Pierce, et al., write, the average of them preserves the unbiasedness, is more accurate and more precise than either of the single-marker estimates, and yields a sex- and age-dependent linear combination of the biomarkers as the estimate of GFR.
Cystatin C, a low-molecular-weight protein synthesized by all nucleated cells, functions as a cysteine protease inhibitor.
“It’s freely filtered by the glomerulus,” Dr. Ferguson said, and unlike creatinine, cystatin C concentration is unaffected by muscle mass, diet, age, or gender. It also has been shown to increase faster than creatinine when eGFR declines—80 mL/min/1.73m2 compared to 40 mL/min/1.73m2 for creatinine. “So that’s a pretty big difference in eGFR,” she said. The main issue with cystatin C is lack of assay standardization; though there is an international certified reference material, not all assay manufacturers have used it for calibration.
In Fig. 2 are the hazard ratios for adverse outcomes using the continuous model of eGFR. “These graphs are showing outcomes—all-cause mortality, cardiovascular, just a whole host of outcomes,” Dr. Ferguson said. Graph A shows the associations of eGFR based on creatinine alone with adverse outcomes. Graph B shows the associations of eGFR based on creatinine and cystatin C. “The eGFR-creatinine graph has more of a J-shaped curve,” she said. “It takes longer and the eGFR decreases more before you get an increase in hazard ratio. When eGFR is calculated with cystatin C and creatinine, it is much more linear and the hazard ratios increase at a lower amount of decrease of eGFR.”
The 2024 KDIGO clinical practice guidelines indicate a number of clinical conditions in which the combined equation is recommended over the creatinine-based equation. These include eating disorders, above-knee amputation, spinal cord injury with paraplegia/paraparesis or quadriplegia/quadriparesis, and others involving body habitus and changes in muscle mass. The guidelines recommend the combined equation when eGFR-creatinine is less accurate and GFR affects clinical decision-making (Stevens PE, et al. Kidney Int. 2024;105[4S]:S117–S314).
In the CYS-A 2024 CAP Survey, there were 333 participating laboratories for all methods of cystatin C, Dr. Ferguson said. “In comparison, for the creatinine Survey there were over 6,000 participants.” Cystatin C is available on some analyzers, “but on others there’s just an option to put it on an open channel,” she said. Of the data on the variation in cystatin C methods, she said: “There’s more variation at lower concentration compared to higher concentration, but there is still quite a bit of variation among the different assays that are available. If you use the cystatin C value in an equation that was generated from data using a different cystatin method, it may not be as accurate as you would like.”
The laboratory at Children’s Mercy wasn’t performing cystatin C in-house when it received the request to implement the equations for eGFR in the LIS. In 2019, send-out testing volumes were low, at 107. “But it was definitely increasing up until 2022,” she said, when 587 tests were sent out. They decided to bring it in-house and went live in November 2022 with the Gentian turbidimetric immunoassay, which is IFCC standardized. “Concentration is matched to a calibration curve to get the result,” she said. The laboratory uses a 0.51–1.05 mg/L reference range.
Cystatin C volumes have risen steadily through May this year, Dr. Ferguson said. “So we’ve not regretted bringing it in-house.”
She and colleagues planned to implement in the LIS three of the CKiD U25 equations: eGFR-creatinine, eGFR-cystatin, and eGFR-average. “Since it’s pretty complicated with all the constants and figuring out the sex and height and age of the patient, we used the rule function in Cerner to help pull the appropriate data and pick the correct constant,” Dr. Ferguson said. “For the creatinine equation we had to pull height from the medical record, the serum creatinine value from the LIS, and then use the sex and age of the patient to calculate the correct constant. Cystatin was a little easier—we didn’t need the height—so it was mainly the laboratory data and then the sex and age.”
When eGFR is ordered, Cerner pulls the most recent creatinine and/or cystatin C result. “If we haven’t had one resulted in the previous two hours, that would fire a reflex order for either a basic metabolic panel or a cystatin C.” If height is required, the LIS pulls the most recent measurement from the prior three months.
The creatinine equation went live first, in May 2022. “You have to order the equation in our system,” she said. “In the adult world, with every creatinine result an eGFR is also reported. But we didn’t want to do that, especially because the equation was developed in patients with mild to moderate CKD, and we didn’t know how valid it was in a healthy population. So you have to take the step and order an eGFR-creatinine.” The cystatin C equation is ordered automatically. “If you’re ordering a cystatin, you’re concerned with kidney function. So we will automatically order the eGFR-cystatin, as well as the eGFR-average, and if you don’t already have a creatinine we’ll do that as well.” The normal range on all equations is greater than or equal to 90 mL/min/1.73m2.
Through May of this year, 2,760 eGFR-creatinine values have been resulted. The eGFR-cystatin and eGFR-average equations went live later, in November 2022; 2,421 eGFR-cystatin values and 1,875 eGFR-average values had been resulted through May.
“Since we have some patients who are getting reports from two different estimating equations, how often did these results not agree?” Dr. Ferguson asked. When both eGFR-creatinine and eGFR-cystatin were ordered, the values were concordant 1,706 times (72 percent) and discordant 675 times (28 percent). Concordance was calculated as both values above or below 90 mL/min/1.73m2. “When we looked at the 675 discordant values, the eGFR-creatinine value was the higher value 519 times. So if you had just done the eGFR-creatinine, you would have gotten a normal result. The eGFR-cystatin was higher just 156 times.”
Did the discordant values change the stage of CKD the patient was classified to have? Stage one CKD is classified as an eGFR greater than 90 mL/min/1.73m2. Stage two is from 60 to 89 mL/min/1.73m2. In 645 of the patients with discordant values, “one result gave them stage one, greater than 90, and then the other result was somewhere between 60 and 89. So it dropped them down one stage. From looking at the numbers, a lot of those values were in the 80s, so not that different if the greater than 90 result is close to 90.” Twenty-five patients, however, had one value greater than 90 and a second value that dropped them down to stage 3a, or between 45 and 59 mL/min/1.73m2. “And five patients had an even greater discrepancy in their values,” with one value greater than 90 and the other value between 30 and 44, classifying the patient as stage 3b.
Said Dr. Weidemann, “In real-world terms, these are children that one might consider flagging as ‘low GFR,’ which would lead to additional interventions—repeat labs versus referral to nephrology, for example. In practice we have many subspecialty clinics such as oncology, bone marrow transplant, hepatology, and cardiology that now routinely perform cystatin C screening as annual risk surveillance labs, and we are starting to see quite a few more of these children earlier in diagnosis.”
Though it was a small number of patients for whom the values were highly discrepant, Dr. Ferguson said, “it’s interesting to look further into the medical record to see if we can come up with a reason why there’s such a big difference between the two values.” Sometimes, she said, the system doesn’t calculate the average of the two equations. “Most often, the reason for that is both GFR values are greater than or less than. So if both are [greater than] 90, you don’t need an average, and same if both are less than 15.” (This is also the reason why fewer averages than eGFR-cystatins have been resulted since the equations went live, she noted.) “There are some cases, however, where it’s random; I don’t know why it didn’t calculate the average.”
In some instances, the system doesn’t automatically order the eGFR-creatinine and the average when eGFR-cystatin is ordered. This occurs when the patient’s lab draw precedes their clinic visit and their height is entered after the lab order is activated, or when the system doesn’t have a height for the patient at all. “So you can’t calculate the eGFR-creatinine.” Similar problems have occurred when height is entered into a field from which laboratory data can’t be pulled, or when the wrong units of height are entered. Another reason the system fails to calculate an eGFR-creatinine and an average is the creatinine value is less than 0.15 mg/dL. Other times it’s just an order error.
“I always like it when I get phone calls from the clinicians because they are the canary in the coal mine,” Dr. Ferguson said. “I wouldn’t know this problem is occurring until they call me.” She recalls an issue with a particular patient. “In investigating, we realized our three-month lookback for height wasn’t functioning as we thought it should—it was going back over 12 months to pull height. In an adult, this might not be much of a problem, but children grow fast. So this could lead to not clinically significant data for that patient at that time.”
It’s a work in progress, she summed up, “but overall it’s been a pretty successful implementation.”
Charna Albert is CAP TODAY associate contributing editor.