One in every ten persons walking on the street has chronic kidney disease, and about a million people die of kidney disease on this globe each year. Chronic kidney disease is a very common condition, but the knowledge of kidney function tests is not. Though poor management of kidney disease plays a vital role in producing these dismal stats, particularly in developing countries, the lack of awareness about kidney disease allows patients to reach this stage in the first place. Understanding how we measure kidney function can be of paramount importance, information that a layperson can use to detect any defect in his kidney function or, even better, a kidney disease patient can use to manage his kidney disease better. What is Serum Creatinine? To understand kidney function, it is essential to learn about creatinine. Creatinine is a muscle breakdown product; your muscles produce a certain amount of this waste daily. Therefore, a fixed load of creatinine filters into the urine, providing a practical tool to measure the filtration rate of your kidney, called the glomerular filtration rate, per minute. The creatinine level in your body depends on your muscle mass. For example, older and sicker patients, whose muscles are usually atrophied, can have lower creatinine levels. Likewise, creatinine levels are generally lower in vegetarians than meat lovers. Creatinine is just a reflection of your kidney function. This fact tells you that creatinine is just a reflection of your kidney function. Lowering the creatinine doesn’t necessarily mean that the kidney function has improved. So why do I mention it? Because regarding creatinine, there is a universal desire, expressed by almost all CKD patients-to lower blood creatinine levels, as if lowering the creatinine in their blood would somehow restore their kidney function. Unfortunately, there is no reason to lower your creatinine levels, and all the natural or home remedies to try to lower your creatinine level can actually further damage your kidney function. What is the Estimated Glomerular Filtration Rate-eGFR Patients can know that they have kidney disease only when they do a blood test called creatinine, usually indicated by Scr (Serum creatinine) in most lab reports. As we learned above, creatinine is a muscle-breakdown product in our blood. Using serum creatinine, a doctor measures the patient’s glomerular filtration rate-an estimated rate of filtration by kidneys per minute. A doctor measures your eGFR by punching the serum creatinine number along with your age and gender in an equation. Small changes in creatinine can make large swings in eGFR. Therefore, doctors often repeat creatinine levels to confirm the decline in your kidney function. And it is because of this variability in lab results that guidelines require the same or a lower eGFR over three months to diagnose CKD. Why Do We Measure eGFR-The Five Stages of CKD Humans like to measure, organize, and classify things. That’s our way of making sense of the world around us-and inside us. Likewise, doctors’ struggle to measure kidney function has been going on for a long time. For example, serum creatinine has been used to measure kidney function since the 1920s. With time, scientists came up with better formulas for accurate and standard measurements of kidney function. Fast forward, a breakthrough came with a simpler version of an equation called MDRD that could use only three parameters-serum creatinine, age, and gender-to calculate eGFR. Afterward, laboratories across the globe started including eGFR in their reports, changing eGFR from a tool for researchers to a key for doctors to manage CKD. We now divide kidney function into five stages based on the eGFR number, stage 1 being normal and stage 5 being the worst kidney function (also called end-stage kidney disease if the patient is on dialysis). Later on, classification for kidney function tests was improved further by adding urine protein levels in the staging of kidney disease. Creatinine Clearance-How is it different from eGFR? Creatinine clearance has been the traditional method to measure kidney function, used before the arrival of eGFR equations. Since the equations for eGFR come from research studies that use patients who are not elderly, very sick, or on a low-protein diet, kidney function derived from these equations may not be accurate when applied to the general population. For these patients, eGFR is better measured by creatinine clearance. To get creatinine clearance, however, a twenty-four-hour urine sample is required, and total creatinine in this sample is measured. Next, creatinine is measured in the blood. Finally, the laboratory calculates the creatinine clearance by dividing the amount of creatinine in the twenty-four-hour urine by the creatinine in the blood. Collecting twenty-four-hour urine is quite cumbersome, making creatinine clearance a less preferred option by most nephrologists. Yet, we continue to use this test when the specific need for it arises. The Take-Home Kidney disease is a chronic condition, meaning it stays with you for the rest of your life. This companionship can be turned into a healthy one if you understand your kidney disease well. And the key to understanding your kidney disease lies in learning about your kidney function tests. You better start now! The writer is an Internist and Nephrologist. He has won Top Internist Award in 2021 and Top Nephrologist Award in 2022 from Michigan, USA.