Kidney and liver function tests explained

Kidney and liver tests appear on almost every blood panel, are frequently mildly abnormal, and their interpretation depends heavily on context that a number alone does not carry.

Kidney function

What is measured and what it means.

Creatinine is a muscle breakdown product cleared by the kidneys, which means muscle mass affects it — a very muscular person has higher creatinine with normal kidney function, and a frail elderly person can have normal creatinine with substantially reduced function.

Estimated glomerular filtration rate is calculated from creatinine, age and sex, and gives a more useful figure.

Urine albumin-to-creatinine ratio measures protein leaking into the urine and is an independent and important marker — kidney disease is staged using both filtration rate and albuminuria, and albuminuria is frequently omitted from testing.

Chronic kidney disease is defined by abnormalities persisting over three months, which means a single result does not diagnose it.

What causes a mildly reduced result

Common and frequently reversible.

Dehydration, which is the most common cause of a transient rise in creatinine.

Acute illness.

Medications: non-steroidal anti-inflammatories, ACE inhibitors and angiotensin receptor blockers, diuretics, and certain antibiotics — with the combination of an NSAID, a diuretic and an ACE inhibitor being a recognised risk for acute kidney injury.

Recent intense exercise, which raises creatinine.

High protein or creatine supplement intake.

And obstruction, which requires prompt identification.

Living with reduced kidney function

The practical measures.

Blood pressure control, which is the single most important intervention for slowing progression.

ACE inhibitors or angiotensin receptor blockers where albuminuria is present, which are protective despite causing a small expected rise in creatinine on starting.

SGLT2 inhibitors, which now have substantial evidence for slowing progression in both diabetic and non-diabetic kidney disease.

Glycaemic control in diabetes.

Avoiding nephrotoxic medications, particularly non-steroidal anti-inflammatories.

Sick-day rules — temporarily stopping certain medications during illness with dehydration — which prevent acute kidney injury and which patients are frequently not told about.

And dietary advice from a renal dietitian where function is significantly reduced, since general dietary advice may be inappropriate.

Liver tests

Which are not really function tests.

ALT and AST are enzymes released when liver cells are damaged.

ALP and GGT relate more to the bile ducts, with GGT also raised by alcohol and by several medications.

Bilirubin, albumin and clotting time are the measures that actually reflect liver function, and they are the ones that matter most when abnormal.

Mildly raised enzymes are extremely common and frequently do not indicate significant disease.

The common causes of abnormal liver tests

In rough order of frequency.

Metabolic dysfunction-associated fatty liver disease, now the most common cause in many populations, associated with obesity, type 2 diabetes and metabolic syndrome — and which can progress to fibrosis and cirrhosis in a minority.

Alcohol.

Medications, including some statins, antibiotics, anti-epileptics and, importantly, paracetamol in overdose and some herbal and bodybuilding supplements which are a recognised cause of drug-induced liver injury.

Viral hepatitis, particularly B and C, which are treatable and frequently undiagnosed — hepatitis C is now curable with direct-acting antivirals.

Autoimmune liver disease.

Haemochromatosis, an inherited iron overload condition that is more common than generally realised and treatable.

Coeliac disease and thyroid disease.

And Gilbert's syndrome, a benign inherited variant causing isolated raised bilirubin, particularly when fasting or unwell, which requires no treatment and causes considerable unnecessary alarm.

Fatty liver

Worth its own section given its prevalence.

It is strongly associated with metabolic syndrome and is now understood as a hepatic manifestation of it.

The main treatment is weight loss, with evidence that a modest percentage reduces liver fat and a larger reduction improves fibrosis.

Physical activity helps independently of weight loss.

Alcohol reduction matters.

Assessment of fibrosis using non-invasive scores or elastography identifies the minority at risk of progression, which is more useful than repeated enzyme measurement.

And cardiovascular disease is the leading cause of death in this group, which means the liver finding is a prompt for overall cardiometabolic assessment.

What to do with an abnormal result

Practical steps.

Ask what the abnormality is and how large.

Ask whether it should be repeated, since transient abnormalities are common.

Review medications and supplements, which is the most commonly identified reversible cause.

Be honest about alcohol, since the assessment depends on it.

Ask what further tests are indicated, since a defined panel exists for persistently abnormal liver tests.

Look at the trend rather than the single value.

And address the modifiable factors, which for both organs are largely the same: weight, alcohol, blood pressure, glucose and medication review.

General information only, not medical advice. Consult a qualified clinician about blood test results, and tell them about all supplements you take.