Kidney disease: introduction

Kidney disease is a term used to include any abnormality of the kidneys. Many diseases can affect the kidneys, and can be described as acute (rapid in onset) or chronic (longer term) kidney disease.

Acute kidney injury

Acute kidney injury (AKI) is a sudden and recent reduction in kidney function and has a wide range of causes. These include stress on the kidneys due to other illnesses or infections, severe dehydration, or the side effects of prescribed or illicit drugs. It is often due to a combination of factors. AKI can improve in a few days or weeks, but sometimes it causes ongoing problems.

AKI definition/classification

The international Kidney Disease: Improving Global Outcomes (KDIGO) AKI guideline is used to define AKI. The definition is based on the level of creatinine in the blood. AKI is defined when one of the following criteria is met: serum creatinine rises by ≥ 26µmol/L within 48 hours; OR serum creatinine rises ≥ 1.5 fold from the reference value, which is known or presumed to have occurred within one week; OR urine output is less than 0.5ml/kg/hr for more than 6 consecutive hours.

Chronic kidney disease

The term 'chronic kidney disease' is used to cover all levels of kidney impairment. These range from a small decline in kidney performance that causes no symptoms and may have no effect on someone's long term health, to a life-threatening condition that requires regular dialysis or a kidney transplant and has a significant impact on daily life. Chronic kidney disease (CKD) is a common long-term condition. The risk of CKD increases with age. CKD does not always cause symptoms, especially in the early stages, and so people may only become aware of problems when their kidney function has fallen to low levels. CKD is divided into several stages based on the level of decline in kidney performance. Stage 1 represents the smallest decline in function and stage 5 the worst.

Why is CKD important?

CKD is common, frequently unrecognised and often exists together with other conditions (such as cardiovascular disease and diabetes). Moderate to severe CKD is also associated with an increased risk of other significant adverse outcomes such as AKI, falls, frailty and mortality. The risk of developing CKD increases with age. As kidney dysfunction progresses, some coexisting conditions become more common and increase in severity. A small proportion of people go on to end stage renal disease (ESRD) and renal replacement therapy (RRT). Not all patients with advanced kidney disease opt for dialysis or transplantation; some choose conservative kidney management (Renal Association, 2014).

CKD definition/classification

NICE kidney disease guidance defines and classifies CKD. CKD describes abnormal kidney function and/or structure present for more than three months, with implications for health. This includes all people with markers of kidney damage and those with a glomerular filtration rate (GFR) of less than 60 ml/min/1.73m² on at least two occasions separated by a period of at least 90 days (with or without markers of kidney damage). The GFR measures how many millilitres of waste fluid kidneys can filter from blood per minute. The classification of CKD has evolved over time. NICE kidney disease guidance recommends classifying CKD using a combination of GFR and albumin/creatinine ratio (ACR) categories.