Calcium is a mineral that plays an important role in muscle movements (muscle contraction), in blood clotting and also in nerve function. However, most calcium in the body is in the bones. Blood calcium and phosphate levels are linked via various mechanisms (see The hyperphosphataemia vicious circle). CKD can cause both a decrease in blood calcium levels (hypocalcaemia) and an increase (hypercalcaemiaAbnormal increase in blood calcium due to a disturbance of calcium-phosphate balance. It can occur in chronic kidney disease as a consequence of excessive parathyroid hormone synthesis and subsequent release of calcium from bones (in bone resorption). Elevated calcium levels are also associated with bone...). Individual analytic laboratories define normal blood calcium levels differently, and these can differ significantly. In most cases however, the level is 2.3–3.0 mmol/l, although some laboratories consider hypercalcaemia to start at 2.75 mmol/l.
Potassium and sodium
Potassium and sodium are also important nutrient mineralsA mineral, in the context of nutrition, is an essential element (one that can only be obtained from the diet), required to perform functions necessary for life in the body.... in the body. In CKD, potassium is usually reduced (hypokalaemia) due to frequent urination and vomiting and sodium is usually increased (hypernatraemia).
- Normal potassium is about 3.0–4.8 mmol/l and normal sodium 145–158 mmol/l.
- Normal blood calcium should be 2.3–3.0 mmol/l.
- Normal blood sodium should be 145–158 mmol/l.
- Normal blood potassium should be 3.0–4.8 mmol/l.
Erythrocytes and haematocrit
Haematocrit is the volume percentage of red blood cells (erythrocytesErythrocytes are red blood cells and contain the blood pigment haemoglobin. Red blood cells are responsible for transporting oxygen from the lungs into the cells of the tissues. They are predominantly produced in the bone marrow of larger bones....) in the stream and is usually lowered in CKD cats. The kidneys are involved in blood formation via the hormone erythropoietinErythropoietin, abbreviated to EPO, is a growth factor that stimulates red blood cell production. EPO can be produced biotechnologically and administered for kidney failure or after chemotherapy..... In CKD, this mechanism is compromised, so fewer red blood cells are produced. This can lead to anaemia, which manifests itself as paleness of the mucous membranes.
In cats the normal erythrocyte count is 5.0–10.0 mio./µl, and the normal haematocrit is 30–45% (0.30–0.45 l/l). Cats are considered to be anaemic at a haematocrit below 27%.
Glomerular filtration rate (GFR)
The glomerular filtration rateThe glomerular filtration rate (GFR) is defined as the volume of fluid that is filtered by the glomeruli of the nephrons of both kidneys per unit of time to produce primary urine. It is, therefore, the chief measure of kidney function. The rate corresponds to... is the volume of blood passing through the nephrons each minute and is therefore the best measure of kidney function. The procedure involves administering the cat a defined amount of a particular marker substance (for example iohexol) that is excreted exclusively, or at least predominantly, via the kidneys. The levels of the substance are then analysed in blood samples collected at specific intervals and in a 24-hour urine sample. The procedure measures how rapidly the kidneys can eliminate the substance (the rate of clearance). As the rate of clearance is directly related to the filtration rate, the GFR can be determined.
CreatinineCreatinine is a breakdown product of muscle metabolism. Its blood level is dependent, among many factors, on age, weight, nutritional status and muscle mass. Hence, creatinine levels in the blood vary from individual to individual. Creatinine is continuously excreted in the urine. Increased blood creatinine... levels are inversely related to GFR. At a higher glomerular filtration rate the nephronsNephrone sind die Filtereinheiten der Nieren.... can clear more creatinine from the blood, and blood creatinine levels drop. At a lower filtration rate less creatinine is cleared and blood creatine rises. Whereas a reduced GFR, precedes an increase in creatinine; there is a delay between the two changes. GFR measurement is, therefore, well-suited for early detection of CKD. SDMASDMA = Symmetrical Dimethylarginine, which is a degradation product of the amino acid arginine and is excreted solely through the kidneys. SDMA is a highly specific biomarker for kidney function in cats. Read more: SDMA test (blood test)... responds to GFR reduction earlier than creatinine. Hence, the SDMA test is also well-suited to early detection, but is a significantly easier procedure than GFR determination.
Blood tests are important!
The blood tests mentioned above are the most important tests for a cat with CKD and are part of its “kidney profile”. In addition, it’s important that a complete blood count is performed to rule out, or to potential health problems or unrelated disorders. These include, liver and thyroid function tests (the T4 level in particular for the former), diabetes tests based on the fructosamine level and a white blood cell differential test (the percentage of each type of white blood cell, for instance neutrophils, in the blood) to identify potential inflammations.
Depending on the underlying disease, trigger or concomitant disease (for instance secondary hyperparathyroidismDurch die bestehende Hypokalzämie (erniedrigter Kalzium-Blutspiegel) bei der chronischen Nierenerkrankung, aber auch bei Leber- oder Darmerkrankungen kann es zu einer unkontrollierten Produktion von Parathormon (PTH) in der Nebenschilddrüse kommen....), further laboratory tests may be necessary, which your veterinarian will discuss with you.