Investigations for CKD

Keep track of your kidney data with PKB

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 eGFR; 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
Haemoglobin – many patients with higher grades of CKD have anaemia shown by a low haemoglobin as part of a full blood count (FBC) – the higher the haemoglobin, the better
Parathyroid hormone (PTH) – reflects much your bones are affected – the lower the better.

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:

Creatinine/eGFR – 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.
Estimated glomerular filtration rate (eGFR; 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

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
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. If moderate-high (>70), this will contribute to deciding on need for a kidney biopsy (see below). It is moderate-high in diabetes and many types of chronic glomerulonephritis.

Mid-stream urine (MSU)

Microscopic (i.e. that you cannot see) blood in urine, 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.

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. More information about them here.

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).

Last Reviewed on 5 April 2024