It is generally accepted that 30% of cats may develop chronic renal azotemia after 9 years of age. Above age 15, it is thought that over 50% of cats will have some form of CKD. In dogs, the prevalence of CKD is accepted to be less than 1% based on a recent U.K. study. Therefore, most of the discussion below will be based on cats and CKD. There are numerous possible causes of CKD in cats, but its exact pathophysiology has not yet been established. There is increasing suspicion that CKD evolves from possible multiple mini active kidney injuries secondary to ischemia or chronic inflammation (IRIS Napa meeting 2016). Unfortunately, as of now we do not have biomarkers sensitive enough to catch these mi-ni-AKIs. However, this may change our future approach to chronic kidney disease in cats, as it may highlight certain preventative strategies.

Traditionally, the diagnosis of CKD has been based on the presence of renal azotemia and inappropriate USG for at least 3 months’ duration. However, this assumes that we have eliminated post renal causes of azotemia (such as ureteroliths) and that the azotemia has been chronic. Alt takes a 75% decrease in nephron mass for azotemia to appear, which is defined as an increase in creatinine (much more stable and predictable) and/or BUN (much less reliable and variable) above established reference ranges. These references ranges can vary greatly from one reference laboratory to another. The development of isosthenuria is not much better in diagnosing CKD, as its appearance signifies a 68–70% decrease in renal function. Also, multiple conditions will affect USG (endocrine disease, afternoon urine sample, diet, etc.).

Symmetric dimethylarginine (SDMA) is a molecule that has become quite important in the early diagnosis and also staging of CKD in cats and dogs. SDMA has no physiologic role in the body. Studies in veterinary medicine have shown that SDMA increases at roughly at 40% of renal dysfunction, and in some cases will detect a 25% in renal function. Using this biomarker will take some getting used to because we are so used to seeing a normal creatinine and USG and concluding there is no CKD! We are now able to diagnose CKD much earlier than before, and this will hopefully lead to new treatments. SDMA is more sensitive than creatinine at detecting CKD especially at earlier stages of CKD.

It is a specific biomarker as well based on studies. In cats with CKD, SDMA shown to increase 17 months earlier; and in dogs an average of 9 months earlier (Hall et al., JVIM 2014). SDMA is not impacted by muscle mass, thereby much more accurate in low BCS geriatric animals. This is very important because most of our geriatric cats have decreased muscle mass. In addition, with CKD there is progressive muscle mass loss and therefore CKD stage may be understated because of the reliance on creatinine. SDMA is stable, and intraday and interday variability negligible. There is no impact from hemolysis, icterus, lipemia.

Once CKD has been diagnosed, it is important to then refer to staging principles. The IRIS staging guidelines have become a mainstay of staging cats and dogs with CKD. They have permitted us to create clear and objective guidelines on how to treat our patients based on creatinine (because it is more stable and predictable than BUN), proteinuria, and hypertension. The IRIS guidelines underline the importance of regular physical exam and lab work for our patients. For instance, proteinuria and hypertension are often silent. If left undiagnosed and untreated, not only will organ damage occur, but CKD also progresses much faster.

IRIS guidelines have been modified in 2015–2016 to reflect the addition of SDMA as both a diagnostic tool and also a staging tool. An SDMA that is persistently above 14 μg/dl is consistent with CKD. This value reflects IRIS stage 1 and 2 patients if the SDMA is below 25 μg/dl. These patients often have minimal to absent clinical signs. SDMA above 25 μg/dl usually indicates IRIS stage 3, and this is important for cats and dogs with creatinine values in stage 2 but that have muscle mass loss. Therefore, these patients have an underestimated renal function and are likely in stage 3 with that SDMA level. This changes their prognosis and treatment recommendations. The same is true for a creatinine above 45 μg/dl. If a cat or dog has a creatinine that puts them in IRIS stage 3 but an SDMA of 45 μg/dl, this pet is actually in stage 4. The treatment recommendations and prognosis vary greatly between these 2 stages (prognosis 778 days for IRIS stage 3 vs. 30–60 days for IRIS stage 4).

Stage 1 has long been a mystery. It was almost impossible to diagnose as there are usually no clinical signs associated with this stage, and our diagnostics tests were not sensitive enough. However, now with SDMA we can diagnose cat in this stage. This has helped us learn a lot more about early stage CKD and discovered that in fact some cats do have symptoms such as mild weight loss. In addition, we are understanding that stage 1 is not a benign state as previously thought. Because of early diagnosis, this is allowing research to advance in early stage treatments (especially diets).

As such, cats in stage 1 may likely benefit from a geriatric-type diet or an early-stage kidney diet. A study by Hall et al demonstrated possible benefits of cats in stage 1 eating an early kidney disease diet, and a similar study done on dogs by the same author also showed a similar benefit. There are multiple reasons why these diets are likely beneficial including decreased phosphorus and increased omega-3 fatty acids.

References

1.  Sparkes AH, Caney S, Chalhoub S, et al. ISFM consensus guidelines on the diagnosis and management of feline chronic kidney disease. J Feline Med Surg. 2016;18(3):219–39.