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Importance of Kidneys
The kidneys are two bean-shaped organs, each about the size of a fist. They are located just below the rib cage, one on each side of the spine. Kidneys are the human body’s filtration system. Healthy kidneys filter about a half cup of blood every minute, removing wastes and extra water to make urine. Kidneys also remove acid that is produced by the cells of our body and maintain a healthy balance of water, salts, and minerals—such as sodium, calcium, phosphorus, and potassium—in our blood. Without this balance, nerves, muscles, and other tissues in our body may not work normally.
What are the biomarkers of Kidney Health?
Blood tests measuring Serum creatinine, Glomerular filtration rate (GFR) and Blood urea nitrogen (BUN) are used to see how well the kidneys are functioning.
Creatinine
A serum creatinine blood test measures the amount of creatinine in our blood. Creatinine is a chemical compound left over from energy-producing processes in our muscles. Healthy kidneys filter creatinine out of the blood. Creatinine exits our body as a waste product in urine. If the kidneys are not working like they should, serum creatinine level goes up. Usually a creatinine level more than 1.2 for women and more than 1.4 for men may be a sign that the kidneys are not working like they should.
Glomerular filtration rate (GFR)
The GFR is a measure of how well our kidneys remove waste, toxins, and extra fluid from our blood. Serum creatinine level, age, and sex are used to calculate the GFR number.
If the GFR is low, the kidneys are likely not working as they should. As kidney disease progresses, the GFR goes down.
Blood urea nitrogen (BUN)
This test measures the amount of urea nitrogen in our blood. Urea nitrogen is a waste product our body makes from the breakdown of protein in the foods we eat. Healthy kidneys filter urea nitrogen out of our blood and it leaves the body through urine. This process helps keep BUN level within a normal range. A normal level of urea nitrogen will depend on age and other health conditions one may have, but usually ranges from 7 to 20. If BUN levels are higher than normal, this may be a sign that the kidneys are not working as well as they should. As kidney disease progresses, BUN level goes up.
Why use eGFR?
Sepsis dataset has both Creatinine and BUN values measured for most patients over a period of time typically every 24 hours or so.
A serum Creatinine (sCr) level of greater than 1.2 for women and greater than 1.4 for men may be an early sign that the kidneys are not working well. However, the “normal” creatinine level can depend on age, race, gender, and body size. What is considered high for one person might be normal for another.
A normal BUN level is between 7 and 20. As kidney function decreases, the BUN level can rise. However, there are other causes of a high BUN that may not mean that there is a problem with the kidneys. For example, certain medications and antibiotics can cause such increases. Bleeding in the stomach and intestines can also cause BUN to rise.
Estimated glomerular filtration rate (eGFR) is calculated using serum creatinine and other factors, such as age and gender. This measurement accounts for the possible differences in creatinine levels between people, in order to find out the correct level of kidney function. In other words, eGFR measurements can help account for certain differences between people that measuring sCr alone does not. However, normal eGFR can still vary, mainly according to age (it can decrease as we get older). The normal value for eGFR is 90 or above. An eGFR below 60 is a sign that the kidneys are not working well. Once the eGFR decreases below 15, one is at high risk for needing treatment for kidney failure, such as dialysis or a kidney transplant.
Because the sepsis dataset contains the age and gender of each patient, it is easy to calculate the eGFR using Creatinine, age and gender. Calculating eGFR can be a better indicator of a patient’s kidney function compared to considering Creatinine or BUN values alone.
Calculating eGFR
Now, Let’s look at how to calculate the eGFR for each patient. The National Kidney Foundation recommends using the CKD-EPI equation for calculating eGFR from Creatinine. The CKD-EPI equation is modeled using least squares linear regression to relate log transformed measured GFR to log-transformed filtration markers, age and sex with two slope splines for creatinine.
eGFR = 142* min(standardized Scr/K, 1)α * max(standardized Scr/K, 1)-1.200 * 0.9938Age * 1.012 [if female]
eGFR (estimated glomerular filtration rate) = mL/min/ 1.73 m2
Scr (serum creatinine) = mg/dL
K = 0.7 (females) or 0.9 (males)
α = -0.241 (females) or -0.302 (males)
min = indicates the minimum of Scr/K or 1
max = indicates the maximum of Scr/K or 1
Now that we know that eGFR is a better indicates a patient’s kidney function, lets categorize the patients based on it.
The below chart shows the various stages of kidney disease and the number of male and female patients in each category. This chart is taking into account only the patients in septic shock.
We see that in each stage of kidney disease, there is a greater number of male patients.
I hope you will agree with me that eGFR is a better indicator of kidney health compared to Serum Creatinine or BUN because it takes into account a patient’s gender and age.
Thanks for reading!
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