How to Monitor Kidney Function for Safe Senior Dosing

When you’re 75 or older, taking a simple pill can become risky-not because the medicine is dangerous, but because your kidneys don’t work the way they used to. About 30% of common medications for seniors are cleared from the body by the kidneys. If those kidneys aren’t working well, the drug builds up. That’s how you get dizziness, confusion, falls, or worse-kidney damage from something meant to help you.

The problem isn’t that doctors don’t care. It’s that most of them are using the wrong tools to check kidney function. Many still rely on outdated formulas or let their electronic health records pick for them. And that’s where things go wrong.

Why Kidney Function Changes with Age

Your kidneys don’t suddenly stop working as you get older. But they do slow down. By age 70, the average person loses about 40% of their kidney filtering capacity compared to when they were 30. That’s normal aging-not disease. The tiny filters inside your kidneys, called nephrons, shrink and die off. Blood flow to the kidneys drops. Muscle mass decreases, which affects creatinine levels-the main marker doctors use to estimate kidney function.

This matters because most drugs are designed for healthy 30-year-old kidneys. If you’re 80 and your kidneys are filtering at 60% of their original speed, and you take the same dose as a younger person, you’re essentially overdosing. Medications like warfarin, digoxin, gabapentin, and even common painkillers like ibuprofen can build up to toxic levels. A 2024 study in Frontiers in Medicine found that using the wrong kidney function estimate led to 25% more dosing errors in seniors taking blood thinners like dabigatran.

The Four Equations That Actually Matter

There are four main ways to estimate kidney function. Only two are reliable for seniors-and even those have limits.

• Cockcroft-Gault (CG): This is the oldest formula, created in 1976. It uses age, weight, gender, and serum creatinine. The catch? It works best if you use ideal body weight, not your actual weight. For frail seniors or those with low muscle mass, using real weight overestimates kidney function by up to 30%. For obese seniors, it underestimates it. Studies show CG with ideal body weight reduces dosing errors by 25% compared to standard CG.
• MDRD: Introduced in 1999, this formula was meant to be more accurate than CG. But it was developed using younger adults. In seniors over 65, it tends to underestimate kidney function by about 1.4 mL/min/1.73 m². That might sound small, but it can push someone into a higher kidney disease stage than they’re actually in-leading to unnecessary dose reductions.
• CKD-EPI: This became the standard in 2009. It’s better than MDRD for most people, especially those with eGFR between 60 and 89. But for seniors over 75, especially those who are thin or malnourished, CKD-EPI still misclassifies kidney function in up to 30% of cases. A 2019 study found it often labels someone as Stage G2 when they’re really G3, which means they get less cautious dosing than they need.
• BIS1 and FAS: These are the new kids on the block. Developed specifically for older adults, they account for age-related muscle loss and other changes. BIS1, for example, correctly estimated kidney function in 95% of patients over 85, compared to just 78% for CKD-EPI. Dr. Sophie Dupont’s team at the University of Michigan found switching from CKD-EPI to BIS1 cut medication-related adverse events by 18% in patients over 80.

Here’s the truth: CKD-EPI is not the best for seniors. It’s the default because it’s built into most hospital systems. But it’s not designed for people who’ve lost muscle, eat poorly, or have chronic illness.

When to Use Cystatin C

There’s another test that doesn’t get used enough: serum cystatin C. Unlike creatinine, it’s not affected by muscle mass. It’s a cleaner signal of kidney function. The problem? It costs $50-$75 more than a basic creatinine test. Many clinics skip it because of cost.

But here’s when it’s worth it: if your creatinine-based eGFR is between 45 and 59 mL/min/1.73 m², but you have no diabetes, no high blood pressure, and no protein in your urine, your kidneys might actually be fine. Your low creatinine could just mean you have low muscle mass-not kidney disease. A cystatin C test can clear that up. If cystatin C says your kidneys are working normally, you might not need to cut your medication dose at all.

Dr. Maria Flamant, lead author of a major 2013 study, said it plainly: “CG with ideal body weight performed better than CKD-EPI in geriatric populations.” That’s still true today.

Real-World Mistakes and How to Avoid Them

Here’s what happens in real clinics:

• A 78-year-old woman with osteoporosis and low appetite has a creatinine of 1.1 mg/dL. Her EHR auto-calculates her eGFR as 58 using CKD-EPI. She’s told to cut her gabapentin dose. But her muscle mass is low. Her BIS1 eGFR is actually 72. She doesn’t need the dose cut-and cutting it leaves her in pain.
• A 92-year-old man with dementia and cachexia is on vancomycin. His creatinine is 1.3. CKD-EPI says his eGFR is 42. He gets a reduced dose. But his BIS1 eGFR is 28. He still gets too much drug and develops kidney injury.

These aren’t rare. A 2023 survey found 78% of pharmacists working with seniors have to override their EHR’s automatic kidney function calculation at least once a week. Why? Because the system doesn’t know if the patient is frail, underweight, or has low muscle mass.

Fixing this starts with asking three questions before prescribing:

1. Is the patient over 75?
2. Do they have low muscle mass, poor appetite, or recent weight loss?
3. Is the medication narrow-therapeutic-index (meaning small changes in dose cause big effects)?

If you answered yes to any of these, don’t trust the EHR’s default. Use BIS1 or FAS. If you can’t get cystatin C, use Cockcroft-Gault with ideal body weight.

What’s Changing in 2025

Things are finally improving. Epic Systems, one of the biggest EHR vendors, now automatically uses BIS1 for patients over 75. The National Institute on Aging has launched a $4.2 million project called SAGE to build better point-of-care kidney tests for seniors. And in 2024, a new equation called CKD2024 was introduced-it combines creatinine and cystatin C with age-specific math. Early results show 15% better accuracy in patients over 80.

The FDA now requires drug labels to include dosing recommendations based on multiple eGFR equations. That’s huge. It means drug makers can’t just assume one formula works for everyone.

But until every clinic has these tools built in, you need to be your own advocate. If you’re a senior-or caring for one-ask your doctor: “Which equation are you using to check my kidney function? Is it the one made for older adults?”

What You Can Do Today

You don’t need to be a doctor to help. Here’s your action plan:

• Ask for your serum creatinine and eGFR numbers at every checkup. Don’t just accept “your kidneys are fine.”
• If you’re over 75, ask if BIS1 or FAS was used. If not, ask why not.
• If you’re on a drug like warfarin, digoxin, or an antibiotic like vancomycin, ask if your dose was adjusted based on your actual kidney function-not the default EHR number.
• Ask if cystatin C testing is an option, especially if your creatinine is borderline and you’re thin or weak.
• Use the National Kidney Foundation’s online eGFR calculator (updated 2023). It lets you compare results from different formulas side by side.

Medication safety in seniors isn’t about doing more tests. It’s about doing the right test. And that means moving past the one-size-fits-all approach that’s been around for decades.

The goal isn’t just to avoid toxicity. It’s to keep seniors active, independent, and pain-free-without unnecessary restrictions on their meds. That starts with knowing what your kidneys are really doing.