Creatinine test: what the result means (CREA)

Creatinine forms in muscle as creatine — an energy-storage compound — is continuously broken down. Its level in blood stays fairly stable and depends mainly on muscle mass, and it's cleared from the body by the kidneys as they filter blood. When kidney filtration weakens, creatinine builds up in the blood, which is why it's the simplest widely used marker of kidney function. On its own, though, creatinine can mislead: a muscular person's normal range sits higher, while an older or leaner person's sits lower even if their kidneys are actually working less well. That's why labs calculate the estimated glomerular filtration rate (eGFR) from creatinine, age and sex — it shows how many millilitres of blood the kidneys clear per minute. Dehydration, intense exercise and certain medications can temporarily raise creatinine, so an unexpected result is worth repeating.

• As a preventive check on kidney function, especially with diabetes or high blood pressure
• Before prescribing medications cleared through the kidneys, or before contrast imaging
• With signs of kidney trouble: swelling, reduced urine output, fatigue, changes in urine
• To monitor chronic kidney disease and how it's progressing
• During pregnancy — when preeclampsia is suspected

A raised creatinine usually points to weakened kidney function — from kidney damage, infection, reduced blood flow to the kidneys, or a blocked urinary tract. It can also rise temporarily from dehydration, muscle injury (rhabdomyolysis) or certain medications, and during pregnancy from preeclampsia. eGFR and a repeat test give a clearer picture than a single creatinine result.

• Kidney disease — damage, failure, infection, or reduced blood flow to the kidneys
• Blocked urinary tract
• Dehydration
• Breakdown of muscle fibres (rhabdomyolysis) after injury or extremely intense exercise
• Pregnancy complications — preeclampsia, eclampsia
• Certain medications that affect how creatinine is cleared

Worth checking alongside a high result: eGFR (a more accurate measure of kidney function than creatinine alone), whether dehydration or heavy exercise affected the test — worth repeating if so, a urine test and urine albumin-to-creatinine ratio to check whether the kidneys are leaking protein, current medications and supplements, and blood pressure and glucose, the two most common causes of kidney damage.

Low creatinine usually doesn't signal kidney disease — it reflects lower muscle mass, from older age, prolonged bed rest, muscle or nerve conditions, or poor nutrition and too little protein; in those cases cystatin C reads kidney function more accurately. Pregnancy can also lower it, since filtration physiologically increases. Worth checking: whether the result fits your build, and whether cystatin C is needed to assess kidney function when muscle mass is low.