YOUR HEALTH — Issue No. 30
One thing I noticed on your lab results — your kidney function dipped.
Most patients hear that and immediately worry. Sometimes that worry is warranted. More often, it is not — and the difference comes down to understanding what the number actually means, what caused it, and what we do next. That is what this issue is about.
Here is the most important thing: one abnormal reading is rarely the whole story. And even when it is something, catching it early is exactly when we have the most room to work.
What we are actually measuring
When I review your kidney labs, I am looking at three values: creatinine, BUN, and eGFR.
Creatinine is a waste product your muscles produce continuously. Healthy kidneys filter it out. When the kidneys are under stress, creatinine builds up in the blood, and the number rises.
BUN — blood urea nitrogen — is another waste product, this one produced when the liver breaks down protein. BUN can rise when kidney function declines, but it is also affected by hydration, protein intake, and other conditions. I interpret it alongside creatinine, eGFR, and the patient’s overall clinical picture rather than relying on BUN alone.
eGFR — estimated glomerular filtration rate — is calculated from your creatinine along with your age and sex. It estimates how well your kidneys are filtering per minute. A normal eGFR is 90 or above.
An eGFR below 60 that persists for at least three months is one way we define chronic kidney disease. But CKD can also be present when eGFR is above 60, if there is other evidence of kidney damage, such as persistent albumin(protein) in the urine. One abnormal result does not automatically mean you have chronic kidney disease. The pattern over time matters.
For patients with diabetes or hypertension, I will also check a urine albumin-to-creatinine ratio, or UACR — even when creatinine and eGFR look normal. Albumin leaking into the urine can be one of the earliest signs of kidney damage, before the filtering numbers change.
Acute vs. chronic — two very different situations
When I see an abnormal kidney number, the first question I am asking is whether this looks acute or chronic. They point in very different directions.
An acute drop — something that happened recently — is often reversible. The most common reasons I see:
Dehydration. If you were not drinking enough water in the days before your lab draw, your creatinine can read higher than it actually reflects.
NSAIDs. Ibuprofen, naproxen, and other anti-inflammatory medications reduce blood flow to the kidneys. Regular use — or even a few days of heavy use — can push kidney numbers in the wrong direction temporarily.
Medication effects. Some blood pressure medications, diuretics, and others can affect how the kidneys filter. This does not mean those medications are harmful — ACE inhibitors and ARBs, for example, can cause an expected early change in creatinine while actually providing important long-term kidney protection. A medication change before a lab draw is always worth noting.
Recent illness. A stomach bug, a fever, or anything that reduced your fluid intake can show up in the numbers.
When I see an acute drop, I look for what may be driving it — dehydration, recent illness, medication effects, or another problem affecting blood flow to the kidneys. The timing of repeat testing depends on how significant the change is and how the patient is doing. A meaningful decline may need reassessment within days, not months. Once the immediate issue has been addressed, continued monitoring helps determine whether kidney function returns toward baseline, or whether an underlying chronic problem is present.
What actually causes chronic kidney disease
The two leading causes of chronic kidney disease in this country are diabetes and high blood pressure. Together, they account for a substantial proportion of cases. Both can damage the kidneys over time — quietly, without symptoms — until the kidneys can no longer filter the way they should.
This is one of the complications I discussed in Issue No. 29 on Type 2 Diabetes. Diabetic kidney disease is common and develops slowly, which is precisely why regular monitoring matters.
Other causes include long-term NSAID use, autoimmune conditions, recurrent kidney infections, and certain inherited disorders. Sometimes no single clear cause is identified.
Kidney function can sometimes improve
A diagnosis of chronic kidney disease does not mean the numbers will keep declining. Many patients with CKD — particularly in earlier stages — remain stable for years when the underlying causes are managed well.
When we find what is driving the decline and address it, kidney function can stabilize and sometimes improve. Better blood pressure and blood sugar control can help protect remaining kidney function and slow further damage. In some patients, kidney measurements improve as these conditions are brought under control.
What does not reverse is long-standing structural damage — scarring that has built up over years of uncontrolled pressure or sugar. That is why catching this early matters. Early CKD is where we have the most room to work.
The stages of CKD
CKD is classified by eGFR, but the stage alone does not tell the whole story. Urine albumin levels also matter when assessing the risk of progression.
Stage 2 — eGFR 60–89, with evidence of kidney damage. Mildly reduced. Usually found incidentally.
Stage 3a — eGFR 45–59. Mild to moderate reduction. This is where I begin watching more closely.
Stage 3b — eGFR 30–44. Moderate to severe reduction. Dietary adjustments, medication review, and closer monitoring become important.
Stage 4 — eGFR 15–29. Severely reduced. Nephrology involvement is appropriate at this stage.
Stage 5 — eGFR below 15. Kidney failure. Some patients require dialysis or transplantation, while others may choose conservative kidney management depending on their overall health and preferences.
How likely is progression?
Most people with stage 3 CKD will not progress to kidney failure. But the risk varies considerably from one person to another. Someone with stage 3a and little or no albumin in the urine may have a very different outlook from someone with stage 3b and significant albuminuria.
What surprises many patients: for most people with stage 3 CKD, the more immediate concern is not dialysis — it is the heart. Reduced kidney function is independently associated with higher rates of heart attack and stroke, which is why CKD is never just a kidney problem. It is a whole-body conversation.
What you can do
Know your numbers. Ask for your eGFR, creatinine, BUN, and UACR at your next visit — especially if you have diabetes or hypertension. Know your baseline. If it changes, you will know it changed.
Stay appropriately hydrated. Dehydration can temporarily reduce blood flow to the kidneys and contribute to an acute drop in function. Drink enough fluids to stay ahead of it — but people with certain heart or kidney conditions may need individualized fluid guidance from their provider.
Avoid NSAIDs when possible, particularly if you have CKD or are at risk for kidney injury. If you take ibuprofen or naproxen regularly, talk with your provider about safer alternatives.
Control what drives CKD. Blood pressure and blood sugar are the two biggest factors. Both are manageable, and both make a meaningful difference in how the kidneys hold up over time.
Get regular labs. You cannot manage what you are not measuring. If your kidney function has dipped, I want to know what it does next — and so should you.
Something worth saying plainly:
A drop in kidney function on a lab result is not a sentence. It is information. Often, there is an identifiable reason — and sometimes it is something we can address or improve. Find what is stressing your kidneys. Remove it where you can. Control what is driving it. Show up for your labs. I will tell you when something is actually happening. Until then, do not carry more worry than the situation deserves.
The member deep-dive this week goes further — the medications now being prescribed specifically to slow kidney disease progression, why SGLT2 inhibitors are changing how we approach CKD even in patients without diabetes, what dietary changes actually matter by stage, and when I refer to nephrology. Plus a patient guide to nephroprotective medications — if you have been told you have chronic kidney disease, review it and ask your provider whether one of these is appropriate for you.
Next week: Flu vaccines — what they actually do, why the effectiveness varies year to year, and the conversation worth having before flu season hits.
Plain Medicine is published for educational purposes only and does not constitute medical advice or establish a patient-provider relationship. Always consult your healthcare provider before making medical decisions.
— Kyle
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