When you’re over 70, taking medication isn’t just about popping a pill. It’s about making sure your kidneys can handle it. As we age, our kidneys slowly lose function-not because something’s broken, but because they naturally shrink and filter less blood. That’s normal. But here’s the problem: 30% of the most common drugs seniors take are cleared by the kidneys. If you don’t adjust the dose, you risk toxicity, falls, confusion, or even hospitalization.
Why Kidney Function Changes with Age
Your kidneys don’t stop working as you get older-they just get slower. By age 70, the average person has lost about 35% of their kidney filtering capacity compared to when they were 30. That’s not disease. That’s aging. Glomeruli, the tiny filters in your kidneys, thin out. Blood flow drops. Muscle mass declines, which affects creatinine levels-the main marker doctors use to check kidney health. This matters because most drug dosing guidelines were created using data from healthy 30-year-olds. A standard dose of warfarin, digoxin, or gabapentin might be safe for a 40-year-old but dangerous for an 80-year-old with reduced kidney function. That’s why simply relying on age or weight isn’t enough. You need an accurate estimate of how well the kidneys are actually working.The Four Main Equations Used to Estimate Kidney Function
Doctors use formulas to estimate kidney function without doing invasive tests. These are called eGFR equations. But not all of them work well for seniors. Here’s what’s actually used-and what you should ask for.- Cockcroft-Gault (CG): This is the oldest one, from 1976. It uses your age, weight, gender, and serum creatinine. The trick? Use ideal body weight, not your actual weight. For frail or obese seniors, using real weight can overestimate kidney function by up to 25%. Studies show CG with ideal body weight makes fewer dosing errors than other formulas for drugs like vancomycin and dabigatran.
- MDRD: Introduced in 1999, it’s built into many lab reports. But it’s designed for people with chronic kidney disease-not healthy older adults. It tends to underestimate kidney function in seniors with normal or mildly reduced function, leading to unnecessary dose reductions.
- CKD-EPI: This became the standard in 2009 and is now used by most hospitals. It’s better than MDRD, especially for people with eGFR between 60-89. But in seniors over 75, especially those who are thin or malnourished, it still misses the mark. One study found CKD-EPI misclassified nearly 1 in 3 older adults into a lower kidney disease stage, meaning they got the wrong dose.
- BIS1 and FAS: These are newer, designed specifically for older adults. BIS1 (Berlin Initiative Study 1) and FAS (Full Age Spectrum) equations were developed after analyzing thousands of seniors. They account for low muscle mass, slower metabolism, and age-related changes in creatinine production. In patients over 80, BIS1 is 95% accurate compared to gold-standard tests-compared to just 78% for CKD-EPI.
Here’s the hard truth: if your doctor uses CKD-EPI and you’re over 75, you might be getting a dose that’s too high. Ask: "Which equation did you use to calculate my kidney function?"
When to Use Cystatin C
Creatinine isn’t perfect. It’s made by muscle. So if you’re thin, bedridden, or have low protein intake, your creatinine levels look artificially low-even if your kidneys are failing. That’s where cystatin C comes in. Cystatin C is a protein made by all your cells, not just muscles. It’s not affected by body weight or diet. It’s a better marker of true kidney function in frail seniors. If your creatinine-based eGFR is in the 45-59 range but you have no signs of kidney damage (like protein in your urine), your doctor should order a cystatin C test. It costs about $50-$75 more than a basic creatinine test-but it can prevent a dangerous overdose.
What the Experts Recommend
The American Academy of Family Physicians says check kidney function yearly if you have diabetes, high blood pressure, or are on kidney-cleared drugs. But that’s the bare minimum. Geriatric nephrologists like Dr. Sophie Dupont at the University of Michigan say: for patients over 75, start with BIS1. If the result is borderline (eGFR 45-59), add cystatin C. If you’re on a high-risk drug like colistin, aminoglycosides, or anticoagulants, get a 24-hour urine collection for creatinine clearance. It’s the most accurate-and it’s worth the extra step. The American Geriatrics Society’s 2021 Beers Criteria doesn’t pick one equation as the winner. Instead, it says: use clinical judgment. Look at the whole picture-weight, muscle loss, diet, mobility, and other illnesses. One 88-year-old might be thin and frail with low creatinine but failing kidneys. Another might be muscular and healthy with the same creatinine level. The numbers alone don’t tell the story.Real-World Mistakes and Fixes
A Reddit user, MedIntern2023, wrote: "My 88-year-old patient got vancomycin toxicity because we used CKD-EPI. Switched to BIS1, and the toxicity stopped." That’s not rare. A 2023 survey found that 63% of pharmacists in senior care have to override their electronic health record’s default kidney function calculation at least once a week. Why? Because most EHR systems still default to CKD-EPI or MDRD-even for 85-year-olds. One common error? Using Cockcroft-Gault with actual body weight. In obese seniors, that can overestimate kidney function by 20-30%. In underweight seniors, it underestimates it. The fix? Always use ideal body weight for CG. If you’re unsure how to calculate it, ask your pharmacist. They’re trained to do this.What You Can Do Right Now
You don’t need to be a doctor to protect yourself. Here’s your action plan:- Ask for your eGFR number. Don’t just accept "your kidneys are fine." Ask: "What’s my exact eGFR, and which equation was used?"
- Check the formula. If you’re over 75, insist on BIS1 or FAS. If you’re underweight or have low muscle mass, ask for cystatin C.
- Review all your meds. Especially anticoagulants, painkillers, diabetes drugs, and antibiotics. Ask your pharmacist: "Is this dose safe for my kidney function?"
- Get a 24-hour urine test if needed. If you’re on a drug with a narrow safety window (like digoxin or lithium), this is the gold standard.
- Update your records. If your doctor uses CKD-EPI and you’re over 75, ask them to switch. Many EHRs now auto-select BIS1 for patients over 75-but not all do.
The Future Is Personalized
In 2024, a new equation called CKD2024 was introduced. It combines creatinine and cystatin C with age-specific adjustments. Early results show 15% better accuracy in people over 80. The National Institute on Aging is funding a $4.2 million project called SAGE to build point-of-care kidney tests that account for muscle loss and frailty. But the biggest shift isn’t in the science-it’s in the mindset. No more "one size fits all." No more letting your EHR pick the formula. Seniors aren’t just old adults. They’re individuals with unique bodies, diets, and health histories. Safe dosing means matching the drug to the person-not the other way around.Resources You Can Use
- National Kidney Foundation’s eGFR Calculator (updated November 2023): Lets you plug in your numbers and compare results from different equations. - American Geriatrics Society’s Kidney Function Assessment Toolkit (released January 2024): Free download with step-by-step guidance for clinicians and patients. - FDA Guidance (2021): Requires drug makers to list dosing recommendations for multiple eGFR equations on labels for high-risk medications.If you’re on more than five medications, or if you’ve had a recent fall, confusion, or hospital stay, get your kidney function checked properly. It’s not just a lab test. It’s your safety net.
How often should seniors get their kidney function checked?
Seniors on kidney-cleared medications or with diabetes, high blood pressure, or heart disease should get tested at least once a year. If you’re over 75, taking multiple drugs, or have signs of frailty (weight loss, low muscle, fatigue), get tested every 6 months. If you’re hospitalized or start a new high-risk drug, check it before and after.
Is creatinine the only test I need?
No. Creatinine alone can be misleading in seniors, especially if you’re thin or have low muscle mass. A cystatin C test is more accurate and should be added if your creatinine-based eGFR is borderline (45-59 mL/min/1.73 m²) and you have no signs of kidney damage like protein in your urine. Together, they give a clearer picture.
Why does my doctor use CKD-EPI if it’s not accurate for seniors?
Most electronic health records still default to CKD-EPI because it’s the official guideline for the general population. Many doctors don’t know about BIS1 or FAS, or they don’t have time to switch. That’s why you need to ask. If you’re over 75, request BIS1. If your doctor says they don’t use it, ask for a referral to a geriatric pharmacist or nephrologist.
Can I use an online calculator to check my kidney function?
Yes, but be careful. Use the National Kidney Foundation’s official calculator (updated 2023). Input your serum creatinine, age, gender, and race. Then try the same numbers in BIS1 if it’s available. Compare the results. If BIS1 gives you a much lower eGFR than CKD-EPI, talk to your doctor. Don’t change your meds yourself-just bring the numbers to your appointment.
What if my kidney function is low but I feel fine?
That’s common. Many seniors with reduced kidney function have no symptoms until they get a drug overdose. Low kidney function doesn’t mean you’re sick-it means your body processes drugs differently. The goal isn’t to fix your kidneys. It’s to adjust your meds so they don’t harm you. Many seniors live well with eGFR as low as 30-40 if their medications are properly dosed.
What to Do If Your Doctor Doesn’t Listen
If your doctor dismisses your concerns about kidney dosing, ask for a referral to a geriatric pharmacist. They specialize in senior medication safety and know which equations to use, which drugs to avoid, and how to adjust doses based on frailty. In the UK, NHS pharmacists can do medication reviews for seniors at no cost. In the US, Medicare Part D offers a free medication therapy management program for people with multiple chronic conditions.Don’t wait for a crisis. If you’re over 70 and on more than five medications, take control. Ask the right questions. Demand the right test. Your kidneys aren’t broken-they’re just aging. But your safety doesn’t have to be.
