From Cysts to Cortex: Interpreting Kidney Ultrasounds

The For Kidneys Sake podcast series is brought to you by Imperial College Healthcare NHS Trust and North West London Integrated Care Board (NWL NHS)

An ultrasound report lands in your inbox. It mentions a cyst. Or a bright kidney. Or “thin cortex.” Or asymmetry. And suddenly, what was meant to reassure becomes a source of anxiety. In this highly practical episode, Prof Jeremy Levy and Dr Andrew Frankle tackle the six most common renal ultrasound findings that trigger GP referrals and explain what actually matters (and what really doesn’t).

From simple cysts and Bosniak classifications to angiomyolipomas, echogenic kidneys, cortical thinning, scars, and asymmetric kidneys, this episode cuts through the noise. The golden rule? Context is everything. Kidney health checks, eGFR, urine ACR, blood pressure, trump scan wording almost every time. Clear, calm, and clinically grounded, this is 15 minutes that could save you hours of unnecessary worry and referrals. Listen in and interpret with confidence.

Top 5 Takeaways

1. Simple cysts are simple - Bosniak 1 or 2 cysts need no follow-up. Reassure and move on. Complex cysts, however, go to urology — not nephrology.

2. Angiomyolipomas rarely matter - If under 40mm, they’re almost always benign and only need one follow-up scan at 12 months. Refer only if >40mm or in women of childbearing age.

3. “Bright kidneys” mean nothing without context - Check eGFR, urine ACR, and blood pressure. If all normal, ignore the scan comment.

4. Thin cortex or scarring is usually congenital - In patients with normal kidney health checks, these findings are benign variants. In younger patients with suboptimal GFR, code as G2 and monitor annually.

5. Asymmetry is common - A 1cm size difference is often physiological. Only worry if there’s uncontrolled hypertension, rapid GFR decline, pulmonary oedema, or significant size discrepancy.

Resource Links:
NICE GUIDELINES [NG203] chronic kidney disease: assessment and management Overview | Chronic kidney disease: assessment and management | Guidance | NICE

Northwest London CKD guidelines for primary care Chronic kidney disease (nwlondonicb.nhs.uk)

The purpose of this podcast is to inform and educate health care professionals working in the primary care and community setting. The content is evidence based and consistent with NICE guidelines and North West Guidelines available at the time of publication.

The content of this podcast does not constitute medical advice and it is not intended to function as a substitute for a healthcare practitioner’s judgement.

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Transcript

Joana Teles 

For Kidney's Sake makes kidney disease management easy. For Kidneys Sake is for primary care clinicians. For Kidney's Sake is nice, consistent, short and sweet. Welcome to For Kidneys Sake brought to you by Northwest London NHS Kidney Care Team.

Jeremy 

So, hello, I'm Jeremy Levy, consultant nephrologist at Imperial College Healthcare NHS Trust.

Andrew 

And I'm Andrew Frankel, a colleague of Jeremy's at Imperial College Healthcare NHS Trust. And welcome again to another episode in our series of podcasts for kidneys sake.

Jeremy 

Yeah, great to be with you again, Andrew. And this, I hope, is to be a really helpful discussion about ultrasound findings, common ultrasound scan findings in relation to kidneys, of course, that cause anxieties for GPs. And we know this is a problem because we get emails and referrals all the time where an ultrasound's been done often for other reasons, not kidney related, but sometimes kidney related. And we're going to cover, well, I think are the six common reports that cause concern, which are the findings of small lumps in the kidney, have been labeled as angiomyolipomas by the radiographer, the finding of bright kidneys, which that radiographers often said indicate medical kidney disease, the finding of slightly thin cortex on the kidney, finding of scarring or an area of scarring in the kidney, cysts, and finally the sixth one, asymmetric kidneys. The report that says the right and the left kidneys are different sizes.

Andrew

Gosh, Jeremy, and we've only got 15 minutes. So you're going to have to give us your...

Jeremy 

⁓ You know we can do it, Andrew. We'd better get this done. 15 minutes, challenge but doable.

Andrew 

Well, let's start by reminding the listeners of when we as nephrologists recommend an ultrasound in a patient with kidney disease. I accept your point that sometimes these reports come out of the blue when you've organised it for something else. But we highlight the need for an ultrasound in our guidelines and broadly there for patients with more accelerated progression CKD, so a sustained drop in GFR by greater than 25%. Anyone with visible or persistent invisible patients who have lower urinary tract symptoms that could relate to obstruction, patients with family history of polycystic kidney disease and who are aged over 20, and anyone with an EGFR who presents out of the blue with an EGFR of less than 30. The problem for primary care is that reporting can highlight a number of abnormalities, some of which are important, many of which are not. Perhaps it's useful to start by emphasising the following, that a clearly obstructed kidney with dilated kidney pelvis is important and needs urgent referral to urology, not nephrology. And certainly any cysts that are labelled by the ultrasonographer as complex also need urology to rule out a kidney cancer, usually needing a dedicated CT or MRI. But any cyst labelled as simple does not need follow-up at all and just reassurance for the patients.

Jeremy 

That's right, Andrew. And actually, you've gone straight in. So cysts, let's start with cysts. But I got to reinforce what you just said. Anything that hints at an obstructed system, so dilated pelvis, needs urgently to go to urology. Some people out there think that urologists only do bladder and prostate. They deal with the kidney as well when there's a surgical matter. But cysts, where it gets a little bit more complicated, but I think it is still straightforward. So exactly as you said.

If the sonographer has said a simple cyst, then it does not need any follow-up of any sort. are benign findings, very common, and there's decades of evidence they need no follow-up. But sometimes the report talks about this thing called the Bosniak classification, and that can cause confusion. So this is a grading system for cysts that's been around for over 40 years and tries to identify cysts that are worrying, or are not worrying by either ultrasound or CT scan. And again, simple cysts, so these are fluid filled holes with no complex features, are sometimes called Bosniak type 1 cysts, and they need no follow up at all. GPs can reassure patients. And I would hope that the sonographer just said it's a simple cyst. If they say a Bosniak class 2 cyst, then those are also completely benign, and their cysts have got a thin.

Jeremy 

Septa in them but no other worrying features and there's clear evidence they do not need any follow-up either. Anything beyond that, so if the sonographer's written a complex cyst or written that there are nodules or lumps or any Bosniak number above two might well need further imaging, and again those should go to urology, not nephrology, because urologists are experts at complex kidney lesions.

Andrew

OK then, Jeremy, so we've done the cysts and the key point is that simple cysts, fine. Bosniak, one or two, simple, do not need follow up. I do wish that radiologists, when they use this terminology, would tell primary care what the follow up should be rather than just leave that hanging. But that is a message for primary care. We didn't mention multiple though and potential for polycystic disease.

Jeremy 

You're actually right and we need to talk about that now don't we? So multiple cysts which may or may not be polycystic kidney disease. So first of all multiple simple cysts are common and common particularly as you get older and older. So one thing will always be is there a family history of either cystic kidney disease or end stage kidney disease or just kidney disease of some sort? And if you've got somebody with a family history of cystic kidney disease or kidney disease that you don't know the cause of and now you found multiple cysts in your patient, they need some clear thought. And I think that that is a person who should be referred to us. That's nephrologists where you've got multiple simple cysts or multiple cysts and a family history. So we can unpick this properly because if it is adult polycystic kidney disease, the inherited condition, that will be important for them and for their families. Now, if there's no family history, people can still have APKD, polycystic kidney disease. So that would be a scan report that says there are multiple cysts. Usually it has a characteristic appearance, but if again, if there is concern or doubt, those should be referred in this case to us as nephrologists because then we can think about further investigations which might be more imaging, might be genetic testing, but it's important and even if the person's older may well be important for their children or grandchildren. So multiple cysts, yes almost certainly a course for referral. If it's just two or three in an older person, that's not polycystic kidney disease. Polycystic kidney disease is lots of cysts in kidneys.

Andrew 

OK, so Jeremy, what about some of the lumps that are reported in kidneys, particularly those labelled as angiomyolipomas? If the ultrasonographer points that out, are we happy with that?

Jeremy

That's an easy one. The answer is yes. So, sonographers and ultrasound is really good at characterising angiomyolipomas. These are small lumps in the kidney that contain a lot of fat and they have a characteristic appearance on ultrasound and they are benign. And very often, almost always in fact, they're found incidentally, and they're almost always very small and under 10 millimetres or one centimetre. So if the report says an angiomyolipoma, look at the size. If it's under 10 millimetres, it is really of almost no significance. And those ones simply need a repeat ultrasound after one year. They only need referral in, and it really shouldn't be to us; it should be to urologists if they're more than 40 millimetres. Four centimetres or in younger women of reproductive age, because of a small risk that they can grow in pregnancy and bleed. The vast majority though are not in young women and are very small. So all they need is a repeat ultrasound in 12 months. If at that point there's been no change, they need no more follow up at all. If they have grown a little bit and they can, even though they're benign, but actually it's uncommon, then they might need an annual scan. But again, the only concern of a lesion labelled as an angiomyolipoma is if it's more than 40 millimetres or in a young woman who might be getting pregnant.

Andrew 

That's really helpful. So 40 millimetres for the threshold for risk for an angiomyelopoma is now fixed in my memory. And also for me, I learnt the emphasis about the women of reproductive age. So one of the things that gets GPs concerned is when the report says, the kidneys are bright or echogenic and they have undertaken a kidney biopsy because they then report there's indication of medical renal disease.

Jeremy 

This one gets me all the time actually and it's really common. Yeah, so exactly that. The report says kidneys look bright on ultrasound or they say look echogenic. Yeah, and they often have that right at the sonographer. It indicates medical renal disease. They've looked at a hazy black and white image on ultrasound. So what on earth does this mean? So in my experience, most of the time a scan report like that does not indicate any significant problem at all. But...It is really important to interpret it in the setting of the patient in front of you, because it, of course, can indicate that there's inflammation in the kidney, which is what makes the kidney appear more reflective, brighter on an ultrasound. So how would the GP or the order of that scan know that? Well, the three things that we bang on about all the time for kidney disease, have they checked a blood test for the crack in the EGFR, the blood pressure? And the urine dip for blood and protein or measure the urine ACR. If an ultrasound report says the kidneys are bright or echogenic, but the blood pressure is normal, the urine's got no blood and no protein and the GFR is normal, then that is an irrelevant report in almost everybody, if not everybody. But if the GFR were to be abnormal or there's proteinuria or hematuria or nuanced hypertension. Then that report may be of significance, telling us there is an underlying kidney problem, as will go with the urine findings and the blood findings. So interpreting that in the context of the patient, their GFR, their urine and their blood pressure. The important caveat would be the person in front of you who's got sort of GFR between 60 and 90, which we've talked about before, is sort of sub-optimal but might be normal. But if you had a younger patient, let's say under 60 years of age, with the GFR say 70, and then an ultrasound report that shows slightly bright kidneys, then that is chronic kidney disease. That GFR is not above 90. So it would be G2 and then with an ACO depending on their ACR. And that patient may need some further thought because it might be they have got something going on. But the critical thing is measure the urine ACR, measure the blood pressure and then have progressive follow up.

 Jeremy 

But for the vast majority of people who've got no abnormalities in their GFR protein or blood pressure, that report does not need any further investigation.

Andrew

Thank you, Jeremy. So bright kidneys always should be seen in the context of the patient's kidney health check. That is the way to do this. And you may occasionally need to code someone as G2 just to make sure you are keeping an eye on those younger patients with suboptimal GFR who've been reported with bright kidneys. Now something I think, again, which we commonly see, which I think is probably a little easier, is that we get reports of patients who have slightly thin or indeed thin cortices or scarring.

Jeremy 

Again, Andrea, also very, very common. And a bit, my last answer, the context is everything. Context is crucial. And almost always a GP will already have this information. They'll have already done a blood test for kidney function, the creatine and the EGFR. And if they haven't, they should do a urine dipstick to look for blood and protein and measure the urine ACR and again, measure the blood pressure of the kidney health check. Because again, if all those three are normal, blood pressure, ACR, urine dip and the EGFR. A report that says slightly thin cortex or renal scars does not need to create any anxiety at all, can be ignored and is almost always likely to be a congenital finding that that's just what their kidneys look like and of no concern. But in a younger person, again, let's say under 60 with a GFR that's suboptimal, I mean, let's say 60 to 90, then that might indicate that they have had previous kidney damage from something that occurred years earlier. And then they will need long-term follow-up. Nothing is urgent, nothing terrible, but they should be again coded, which would be a G2 code if the GFR 60 to 90, because they've got an abnormality on the ultrasound. And that also helps with an annual check of their kidney health. But again, if the GFR were above 90 and the report says a kidney scar or thin cortex, with no proteinuria, this is of no concern. Scars are slightly different. The sonographer, the person doing the ultrasound has seen a change in the cortex that they've interpreted as a scar. And in almost all those patients is actually a congenital abnormality of the kidney. It was there essentially from birth. And these are just variants of normal. But again, interpreted in the setting of the GFR and the urine ACR.

Andrew

Very happy with that explanation Jeremy, but just to be devil's advocate here, isn't it true that patients with embolic disease can present with scars on their kidneys?

Jeremy 

You're absolutely right, Andrew. of course that's okay. So atheroembolisation or people with thrombotic events who've thrown off an embolus into the kidney can of course get a scar and that can be significant. It's actually incredibly rare unless somebody's had an acute illness. You know, they're in hospital with endocarditis or something similar. So yes, it can happen, but it's very rare. And we do sometimes do a hunt for causes of or procoagulants or inflammatory disease but this is incredibly rare and not common and not common for scans that are done routinely so so you are absolutely right but it's very

Andrew 

Great Jeremy, well, we've covered cysts, echogenic or bright kidneys, scars, thin cortices, angiomyolipomas, and now asymmetric kidneys. When should primary care be concerned at all about a report describing asymmetric kidneys?

Jeremy 

We're all asymmetric, Andrew. My right ear is slightly larger than my left left ear or whatever it happens to be. So first of all, asymmetry is not uncommon to a degree. And again, context is all important. The patient's got uncontrolled hypertension on three drugs. If they've got acute pulmonary edema, if they're rapidly losing kidney function, then asymmetric kidneys may be pointing to renal artery stenosis.

And that would be important. In that context, ⁓ declining kidney function rapidly, poor blood pressure control, even a one centimeter difference between two kidneys might be of significance with the smaller kidney being ischemic and driving those problems. But that's in that context of uncontrolled hypertension, rapid loss of GFR or pulmonary edema. Other than that, a one centimeter difference might be completely physiological.

If there's a bigger difference in the kidney size, two or three centimeters, or even if one kidney is labeled as atrophic, again, it depends on the context. once again, measure the EGFR, measure the blood pressure and measure the proteinuria. Because if the ACR is zero and the blood pressure is normal or well controlled on one drug and the EGFR is normal, then this asymmetry is of no consequence.

Jeremy 

Likely congenital and needsno follow-up. If the report has said one very small kidney and the patients get worried mostly we just need to reassure them but if they're losing GFR with one very small kidney sometimes we might want to see them but we would certainly want very aggressive management of cardiovascular risk factors.

Andrew 

So that's really helpful, Jeremy. You provided so much simple advice. My three takeaways from this are going to be, Firstly, complex cysts or concern about lumps in the kidney need urology referral, not nephrology.

Angiolipomas are only an anxiety if they are more than 40 millimetres in diameter or if they have occurred in a young woman who might get pregnant. When you get reports of, bright kidneys, slightly thin you need to interpret this in the context of kidney health. If all of these are normal, then just ignore the report. If they are younger, and the GFR is suboptimal, then label them G2 and give them an annual kidney health check to monitor them. And finally, again, asymmetry needs to be interpreted in the context of clinical history and kidney health. If it is an isolated finding, it too can usually be ignored and likely congenital.

Jeremy 

Very well, we've done it, Andrews. Six common ultrasound findings in 15 minutes. We could have just used your summary and then it would have been six common ultrasound findings seconds.

Andrew 

Definitely not Jeremy, that's been 15 minutes of your pure gold and I hope our listeners have enjoyed this and found it helpful. And please look out for all our previous episodes and join our mailing list and send in any requests for future episodes.

Jeremy 

That's right. Great to chat to you again, Andrew. Have a great day.

Joana Teles
Thank for listening. We hope you enjoyed this episode. All information is fully consistent with NICE and Northwest London guidelines. You can find out more in the show notes and contact us with any suggestions or questions. Send us a text using the text function at the top of your show notes. Thank you for listening to For Kidneys' Sake podcast and we see you at the next episode.

Joana Teles
Thank for listening. We hope you enjoyed this episode. All information is fully consistent with NICE and Northwest London guidelines. You can find out more in the show notes and contact us with any suggestions or questions. Send us a text using the text function at the top of your show notes. Thank you for listening to For Kidneys' Sake podcast and we see you at the next episode.