Further blood tests


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 (hypercalcaemia). 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 minerals 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 (erythrocytes) in the stream and is usually lowered in CKD cats. The kidneys are involved in blood formation via the hormone erythropoietin. 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 rate 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.

Creatinine levels are inversely related to GFR. At a higher glomerular filtration rate the nephrons 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. SDMA 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 hyperparathyroidism), further laboratory tests may be necessary, which your veterinarian will discuss with you.