Kidney Blood Test Kit

Creatinine is a waste product generated from the normal breakdown of creatine phosphate in muscle tissue. It's produced at a fairly constant rate (depending on your muscle mass) and is filtered out of the blood exclusively by the kidneys, making it one of the most useful markers of kidney function. Unlike some other waste products, creatinine isn't significantly reabsorbed by the kidneys or secreted by other routes—virtually all of it ends up in urine. Because the kidneys are responsible for clearing creatinine from the blood, when kidney function declines, creatinine accumulates and blood levels rise. This makes serum creatinine a reliable indicator of how well the kidneys are filtering. However, creatinine levels are also influenced by muscle mass—people with more muscle naturally produce more creatinine, so a muscular person may have higher creatinine levels that are completely normal for them. This is why creatinine is used alongside eGFR (which accounts for age and sex) and other markers rather than interpreted alone. Normal creatinine levels vary by age, sex, and muscle mass, but are typically 60-110 µmol/L for men and 45-90 µmol/L for women. Elevated creatinine may indicate reduced kidney function, but can also be caused by dehydration, recent intense exercise, high protein intake, or certain medications. Persistently elevated creatinine warrants further investigation. Very low creatinine is less common but can occur with reduced muscle mass, severe liver disease, or pregnancy. Results outside the normal range may need a follow-up with your GP.

The estimated Glomerular Filtration Rate (eGFR) is calculated from your creatinine level, age, and sex to provide an estimate of how much blood your kidneys are filtering per minute. The glomeruli are tiny filtering units within the kidneys (about a million in each kidney) where blood is filtered and urine formation begins. GFR is considered the best overall measure of kidney function. eGFR is reported in mL/min/1.73m² (millilitres per minute, normalised to body surface area). The calculation accounts for factors that affect creatinine levels independent of kidney function—for example, men typically have higher creatinine than women due to greater muscle mass, and creatinine naturally rises with age as kidney function gradually declines. The most commonly used formula is the CKD-EPI equation, which gives a more accurate estimate than older formulas, particularly at higher (more normal) GFR values. Normal eGFR is above 90 mL/min/1.73m². An eGFR between 60-89 is mildly reduced and may be normal for older adults, but should be monitored. Sustained eGFR below 60 for more than 3 months defines chronic kidney disease (CKD). CKD is staged by eGFR: Stage 3a (45-59), Stage 3b (30-44), Stage 4 (15-29), and Stage 5 (<15, which may require dialysis or transplant). Importantly, a single eGFR reading should not be used to diagnose CKD—results should be confirmed with repeat testing, as eGFR can fluctuate day to day. Kidney function naturally declines with age (roughly 1 mL/min/year after age 30-40), so an eGFR of 70 in a healthy 75-year-old may be entirely appropriate. Results outside the normal range may need a follow-up with your GP.

Urea (also called blood urea nitrogen or BUN in some countries) is a waste product formed in the liver when protein is broken down into amino acids and then metabolised. The nitrogen released during this process is converted to urea, which travels through the bloodstream to the kidneys, where it's filtered out and excreted in urine. Urea reflects both protein metabolism (how much protein you're breaking down) and kidney function (how well the kidneys are eliminating waste). Unlike creatinine, which is produced at a relatively constant rate, urea levels are influenced by dietary protein intake and the body's metabolic state. Eating a high-protein meal can temporarily raise urea levels, while a low-protein diet or liver disease can lower them. This makes urea somewhat less specific for kidney function than creatinine, but it provides complementary information—looking at the ratio of urea to creatinine can help distinguish between different causes of elevated results. Normal urea levels are typically 2.5-7.8 mmol/L. Elevated urea can indicate reduced kidney function, but is also commonly caused by dehydration (a very common cause), gastrointestinal bleeding (digested blood provides a protein load), high-protein diet, certain medications (steroids, tetracyclines), or conditions causing increased protein breakdown (burns, trauma, fever). A disproportionately elevated urea compared to creatinine often suggests dehydration or gastrointestinal bleeding rather than kidney disease. Low urea can occur with liver disease (impaired urea production), low-protein diet, or pregnancy. Results outside the normal range may need a follow-up with your GP.

Sodium is both an essential electrolyte and a mineral that plays crucial roles in maintaining fluid balance, blood pressure regulation, nerve impulse transmission, and muscle contraction. The body tightly regulates sodium levels within a narrow range through a complex system involving the kidneys, hormones (particularly aldosterone and antidiuretic hormone), and thirst mechanisms. The kidneys are central to this regulation—they adjust how much sodium is excreted or retained based on the body's needs. In the context of kidney function testing, sodium provides information about the kidney's ability to maintain electrolyte balance and regulate fluid status. Healthy kidneys can adjust sodium excretion over a wide range to match intake and maintain balance. When kidney function is impaired, this regulatory capacity may be diminished, potentially leading to sodium imbalances. Sodium levels can also be affected by many other factors including fluid intake, hormonal conditions, medications (especially diuretics), and various medical conditions. Normal sodium levels are 136-145 mmol/L. Low sodium (hyponatraemia, below 136 mmol/L) can cause symptoms ranging from mild (headache, nausea, confusion) to severe (seizures, coma) depending on how low and how quickly it falls. Common causes include excess fluid intake, certain medications (especially diuretics, antidepressants), heart failure, liver cirrhosis, and syndrome of inappropriate ADH (SIADH). High sodium (hypernatraemia, above 145 mmol/L) is most commonly caused by dehydration, but can also occur with diabetes insipidus or excessive salt intake. Sodium imbalances often reflect problems with water balance rather than actual sodium excess or deficiency. Results outside the normal range may need a follow-up with your GP.