The Silent Killer (diagnosis)

Our prior post provided an introduction to Chronic Renal Failure (CRF or CKD) in dogs and cats and discussed some of the common symptoms. CRF is a silent killer of our beloved pets and often is not diagnosed until the disease has caused great damage to the kidneys. Today's post discusses the clinical diagnois of CRF. Though some technical terminology is used to provide accuracy and substance, the material does not require any specific knowledge to understand.

This is part two of a multipart entry on the subject of Chronic Renal Failure in dogs and cats.

Clinical Diagnosis

To properly diagnose either ARF or CRF, blood and urine samples are analyzed. Standard testing includes a complete blood count, a biochemical profile and urinalysis. The vet may also check the pet's blood pressure, protein to creatinine ratio and the amount of parathyroid hormone. Many vets are able to do these tests in-house so results are available the same day.

The three items the vet will be most interested in are BUN (blood urine nitrogen), creatinine and phosphorus levels. All are likely to show significant elevation as CRF progresses and will be very high in late stage CRF. Sodium, potassium and magnesium (electrolytes) may also be checked for irregularity.

"Staging" is just a clinical term that means 'to identify to what point a disease has progressed'. Diseases are often split into different stages (or levels) to indicate changes in the types of symptoms seen and severity of damage to the body over time. Low numbers indicate the onset of the disease, the fewest symptoms and least damage with the numbers increasing to the final, worst case. The table below defines the four accepted stages of CRF per the International Renal Interest Society (IRIS).

CRF Staging
Stage Serum Creatinine (mg/dl) Serum Creatinine (μmol/L)
1 <1.4 (dog) <125 (dog)
<1.6 (cat) <140 (cat)
2 1.4-2.0 (dog) 125-179 (dog)
1.6-2.8 (cat) 140-249 (cat)
3 2.1-5.0 (dog) 180-439 (dog)
2.9-5.0 (cat) 250-439 (cat)
4 >5.0 (dog/cat) >440 (dog/cat)

CRF Staging Cont'd
Stage Additional Notes
1 Shows a mild renal abnormality such as an increase in proteins in the urine or decreased concentrating ability
2 Some signs of excess urea, creatinine and/or proteins in the urine. May show signs of increased dilute urine
3 Excess urea, creatinine and/or proteins in the urine. Many systemic signs of CRF present
4 Very high urea levels, excess proteins may be present, decreased urinary concentrating ability. Systemic clinical signs are severe.

Excess protein levels ("proteinuria") occur at a protein/creatinine ratio > 0.4 (cats) or 0.5 (dogs). Levels below 0.2 are normal and in between is considered borderline. However, testing with basic urine dipsticks may sometimes give rise to false positives so additional screening is appropriate in those cases.

Blood pressure over 160/100 is indicative of moderate risk, 180/120 is high risk of organ damage when persistent and seen in conjunction with other CRF indicators. Existing hypertension will increase complications. To be useful, readings should be taken multiple times over multiple days.

Proteinuria and blood pressure results are then used to "sub-stage" the dog or cat's condition.

A valuable test that can give indications of CRF at an earlier stage measures the amount of SDMA (Symmetric dimethylarginine). Meta-analysis in human studies indicate SDMA exhibits some properties of a reliable marker of renal function 1. Studies in pets show knowledge of irregular SDMA levels can provide nearly 18 months earlier warning of CRF in cats and 9 months in dogs.

As with cancer, the earlier CRF is detected the better the chance the disease can be slowed and the pet's quality of life extended. Another potential early warning indicator in dogs is periodontal disease. A study in 2011 showed an association between increasingly severe periodontal disease with high serum creatinine and blood urea nitrogen levels (this does not mean to say dental disease is the cause of CRF).

The stage and sub-stage will, in consideration of any other health conditions, indicate what course of treatment is likely to best reduce the symptoms and damage from CRF in order to extend a pet's quality of life and expected life span.

As mentioned previously, there is no cure for CRF. But if caught at a relatively early stage, modification of diet can extend the life of a pet many months, with studies showing an average of around 13 months.

1. Symmetric dimethylarginine (SDMA) as endogenous marker of renal function—a meta-analysis

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