24 Mar 2014
Sarah Caney
Job TitleSARAH CANEY discusses how to achieve early diagnosis of renal disease in cats, IRIS staging as an assessment and monitoring method, as well as when to prescribe
CHRONIC kidney disease (CKD) is one of the most common diagnoses made in clinical practice, and an important cause of morbidity and mortality in the older cat. Earlier diagnosis is desirable to allow intervention with appropriate treatments, which may help to prolong life and enhance quality of life.
The International Renal Interest Society (IRIS) has devised helpful guidelines for assessment and management of CKD.
Making an early diagnosis of CKD means diagnosing this condition in a cat that is not showing clinical signs. Clinical signs are usually expected in cats that have lost at least three-quarters of their functioning nephrons. Evidence of CKD – such as a reduced ability to concentrate urine – may be present in cats that have lost two-thirds of their functional nephrons. For many cats with CKD, clinical signs of illness are not apparent until the disease is relatively advanced (for example, IRIS stage three or four).
Proactive testing is required to make an early diagnosis of CKD. This includes adopting life stage-appropriate preventive health care protocols that “screen” for CKD and encouraging owners to contact their vet should they notice any changes in their cat’s behaviour or health.
The author recommends clinicians follow International Cat Care’s WellCat guidelines.
• Cats of all ages should be assessed at a veterinary practice at least once a year and their weight and body condition score recorded, in addition to a general physical examination and discussion of appropriate preventive health care. In addition to this:
• “Mature” cats – those aged seven years and older – should have their blood pressure (BP) checked once a year and a urinalysis performed. A urine specific gravity (USG) less than 1.035 should be taken as a potential indication of underlying disease, with the most common causes being CKD, hyperthyroidism and diabetes mellitus. Therefore, in cats with a USG less than 1.035, further investigations including a urine dipstick (diabetes mellitus), haematology, serum biochemistry and total thyroxine (T4) should be performed. Further testing (for example, urine culture and urine protein to creatinine ratio) may also be indicated.
• “Senior” cats – those aged 11 years and older – should have blood tests done (haematology, serum biochemistry and total T4) once a year, in addition to the recommendations already made for mature cats. Trends showing progressively increasing creatinine results may be an early indication of CKD.
• “Geriatric” cats – those aged 15 years and older – should be assessed at a veterinary practice every six months, at which time a clinical examination, weight check, body condition score, BP and urinalysis should be performed. Blood tests should continue to be done annually unless there is any clinical indication to increase the frequency of these.
Measurement of glomerular filtration rate (GFR) is not standard in clinical practice, but as availability and practicalities of this improve, this may change.
The IRIS was formed in 1998 and comprises a panel of 16 veterinary nephrologists from around the world. IRIS has established an internationally recognised set of guidelines concerning diagnosis, assessment of severity (“staging”), monitoring and treatment of cats and dogs with renal disease (www.iris-kidney.com).
The classification of CKD into clinical stages can be very helpful in guiding management and is done once a diagnosis is obtained.
Patients are primarily staged according to their blood creatinine levels (Table 1).
It is important to use creatinine values obtained after the cat has been rehydrated and it is recommended two samples are collected, with at least two weeks between these samples.
Some cats with CKD will pass through all of the IRIS stages as their kidney disease progresses; other cats will remain stable for many years in the same stage. Further classification can then be made on the basis of presence of proteinuria and/ or systemic hypertension.
The IRIS stage has some value in predicting prognosis – in one study median survival from the time of diagnosis was 1,151 days (range two to 3,107) for cats in stage two with creatinine levels above 200µmol/l, 778 days (range 22 to 2,100) for cats in stage three and 103 days (range one to 1,920) for cats in stage four CKD (Boyd et al, 2008).
Urine normally contains small amounts of albumin in addition to proteins lost from the tubules (Tamm-Horsfall protein) or lower urinary and genital tracts. Proteinuria (abnormal loss of protein in the urine) can result from renal, prerenal and post-renal causes.
• Pre-renal causes: proteinuria caused by filtration and excretion of proteins that are not normally present in the circulation – for example, haemoglobin and myoglobin. History, physical examination and routine blood work can help to rule out pre-renal causes.
• Renal causes: proteinuria can result from physiological and pathological processes.
– Physiological causes – for example, pyrexia, strenuous exercise, seizures and stress. These are usually a cause of mild, transient proteinuria.
– Pathological reasons – for example, glomerular disease, tubular disease and interstitial disease. Proteinuria is an important potential complication of renal disease and may be mediated by angiotensin two. It is now recognised presence of proteinuria is not only a marker of renal injury, but also may be an important independent mediator of progressive renal injury.
• Post-renal causes: entry of protein into the urine after it has passed through the renal pelvis – for example, lower urinary tract disease. Sediment examination and urine bacteriology are important to rule out post-renal causes.
Although dipsticks can detect protein in the urine, they are relatively insensitive and thus often miss mild, but still significant, proteinuria. The urine protein to creatinine ratio is considered to be the gold standard test for assessment of proteinuria. Pre and post-renal proteinuria should be ruled out before diagnosing renal proteinuria. A cystocentesis urine sample is preferred when assessing proteinuria in CKD patients.
IRIS guidelines for interpretation of results in cats with CKD (IRIS stage two, three and four) are shown in Table 2.
Repeat assessment is indicated within two to four weeks to confirm any proteinuria documented is persistent and of similar magnitude. If this remains the case then treatment with an angiotensin converting enzyme inhibitor (ACEI) or angiotensin receptor blocker (ARB) is indicated.
Hypertension is classified according to the risk of target organ damage (TOD) as in Table 3. Patients are sub-classified according to whether they have evidence of target organ damage. Blood pressure should be measured using Doppler methodology.
Treatment of hypertension is recommended for cats with persistently abnormal systolic blood pressure (SBP; SBP greater than 160mmHg) and/or where evidence of TOD such as retinal haemorrhage or detachment is detected (Brown et al, 2007). Systemic hypertension can usually be managed effeceffectively using either amlodipine (under the cascade), an ACEI or both simultaneously.
In cats with kidney disease, phosphate levels should be kept as close to the bottom of the reference range as possible to reverse/prevent development of renal secondary hyperparathyroidism. IRIS recommends all cats in IRIS stage two, three and four CKD are “phosphate restricted” through use of a renal food and/or oral phosphate binder.
IRIS has published guidelines on phosphate levels in cats with CKD, according to their stage of disease (Table 4). Cats with phosphate levels above the IRIS target ranges should have their phosphate restriction intensified – for example, through addition of an oral binder to a phosphate restricted diet.
Patients with CKD may benefit from a number of additional treatments not discussed in this article, such as appetite stimulants, anti-nausea and anti-emetic agents, subcutaneous fluid therapy and so on (Table 5). Renal prescription diets are especially proven to provide clinical and survival benefits to cats in IRIS stage three and four CKD (Plantinga et al, 2005; Ross et al, 2006).
Early diagnosis of CKD is a challenge, but in proactively diagnosing this condition we hope to enhance quality and length of life for affected patients. The IRIS guidelines offer useful support in staging the severity of disease and providing treatment recommendations.
Regular examinations, including weight checks, are a good way of detecting insidious illnesses such as chronic kidney disease (CKD).
Table 1. IRIS staging of chronic kidney disease (CKD)
Table 2. IRIS guidelines for interpretation of results in cats with CKD (IRIS stage two, three and four)
Sarah Caney
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