Tests and Investigations for CKD
Key Points
- CKD is not a diagnosis. It is a syndrome with 7 groups of causes. Using the investigations below, you need to ascertain the underlying cause as this guides treatment
- Blood creatinine level – is the most important test. It is put into an equation to make another number called GFR; the lower the creatinine and higher the eGFR, the better
- Blood potassium level – potassium (high or low) can affect the heart function, and so should be addressed – normal is ideal
- Urine albumin-to-creatinine ratio (ACR) – indicates the amoint of protein in the urine – higher levels reflect strain on the kidneys
- Renal (kidney) ultrasound – tells the doctor if you have 2 kidneys (1 in 1000 have one) and if they look normal.
So. Let’s now go through investigations for CKD. When doctors investigate whether you have CKD, there are ..
“Three big questions to ask, and three big tests to do.”
Three big questions
- What is the kidney function?
- Are the kidneys leaking protein? If it is a lot, the disease may be in the glomeruli
- Do you have 2 kidneys and what do they look like?
Three big tests
To answer these three big questions, people with CKD, will need three big tests. They are:
- GFR/Creatinine – the most important one. See below
- Urine albumin-to-creatinine ratio (ACR). See below
- Renal (kidney) ultrasound. See below.
Here are all the blood, urine and x-ray tests that need to be done for most patients with CKD3B, CKD4 or CKD5. Most patients with CKD1-2 or CKD3B will not require all of these tests.
Blood tests
There are four groups of blood tests: haematology, biochemistry, bone biochemistry and immunology.
Haematology
Full blood count (FBC)
- Haemoglobin (‘Hb’). Red cells, carry oxygen. The normal range of Hb = 130-170 g/L (for men), and 110-150 g/L (women). A low Hb is called anaemia, and is a feature of more severe CKD
- White cells = 4-11. Fight infection. Normal in CKD
- Platelets = 150-400. Clot blood. Normal
Ferritin
>100 mcg/L. Iron stores. Normal
Biochemistry
U+E (urea and electrolytes)
- Sodium = 135-145 mmol/L. Mineral/salt. Variable in CKD, usually normal
- Potassium = 3.5-5.3 mmol/L). Mineral/salt. Normal or high (occasionally low). If high (or low), heart function can be affected
- Urea = 3-7 mmol/L. Waste product. High
- Creatinine = 60-120 mcmol/L. Waste product. One of ‘big three’ key investigations. High (‘has to be’ as part of making a diagnosis of CKD). A value above 300 mcmol/L shows significant CKD and you need it measuring at least every 2 months.
- Glomerular filtration rate (GFR; based on creatinine; kidney function) = 90-120 mls/min. Low in CKD (again by definition). A value below 30 ml/min shows significant CKD and you need it measuring at least every 2 months
- CKD Stage
- The 5 stages of CKD are based on the blood GFR
- The higher the GFR, the better the kidney function (and therefore the lower stage of CKD that the patient will be in).
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Glucose = 4-6 mmol/L. Diabetes (high). Normal in CKD
Bicarbonate = 22-28 mmol/L. Acid levels. Low. Decreasing bicarbonate levels can be a sign dialysis is needed
Liver function tests (LFTs). Liver function. Normal
Cholesterol (non-fasting). Normal
Bone biochemistry
- Calcium = 2.2-2.6 mmol/L. Bone health. Low in CKD
- Phosphate = 0.8-1.4 mmol/L. Bone health. High
- Alkaline phosphatase = 50-150 iu/L. Bone health. Normal or high
- Parathyroid hormone (PTH) = <5 pcmol/L. Bone health. High
- Vitamin D. Bone health. Low
Immunology
- Anti-nuclear antibody (ANA). Positive in systemic lupus erythematosus (SLE, lupus). In most CKD patients, it will be negative
- Anti-neutrophil cytoplasmic antibody (ANCA; with PR3 and MPO). Positive in vasculitis. In most, negative
- Complement C3/4. Low in SLE. In most, negative
- Double-stranded DNA (DsDNA). Positive in SLE. In most, negative
- Serum electrophoresis. Done looking for an abnormal protein suggestive of myeloma (and other blood disorders). In most, negative
- Immunoglobulins (IgG, A and M). IgA high in 50% people with IgA nephropathy. In most, negative
- Serum free light chains (SFL). Done looking for myeloma (and other blood disorders). In most, negative.
+/- Other blood tests
- Anti phospholipase A2 receptor (PLA2R) antibody. Positive in primary membranous nephropathy, one type of chronic glomerulonephritis. In most, negative
- Hepatitis B and C, HIV. Can cause several types of chronic glomerulonephritis. In most, negative
- Prostate specific antigen (PSA). Male only. Raised in prostate cancer. In most, negative.
Urine tests
Urinary ACR (uACR)
= < 3 mg/mmol. One of ‘big three’ key investigations.
- Measures the level of a protein called albumin in urine
- Higher levels indicates strain on the kidneys
- If moderate-high (>70), this will contribute to deciding on need for a kidney biopsy
- It is moderate or high in diabetes and many types of chronic glomerulonephritis.
Mid-stream urine (MSU)
Microscopic (i.e. blood in the urine you cannot see) – some types of glomerulonephritis and urinary tract infection (UTI).
Imaging (x-rays etc)
Renal ultrasound
One of ‘big three’ key investigations. Assesses number (two) and size (10-14 cm long) of kidneys; and may give the cause of CKD, especially polycystic kidney disease, and obstruction (blockage) of the kidneys
CT scan
A minority of patients will need a CT scan rather than an ultrasound. There are advantages to these (better picture) but also disadvantages (can worsen CKD, sometimes permanently).
Other investigations
Kidney biopsy
A small number of patients need a kidney (renal) biopsy. This is a procedure to take a sample of kidney tissue, about the size of a small pin, which is examined under the microscope. It is usually done to make a diagnosis of a chronic glomerulonephritis, and in nephrotic syndrome.
Summary
We have described investigations for CKD. We hope you understand them better now.
Top Tip
A eGFR under 30 ml/min (stage 4 CKD) – or a creatinine above 300 mcmol/L – shows significant CKD and you need it measuring at least every 2 months; and to be under the care of a kidney specialist (nephrologist).
