Kidney Function

Sodium is one of the most important electrolytes in your body, and this blood test measures its concentration in your bloodstream. It is central to regulating the balance of fluids inside and outside your cells, supporting nerve signalling, and helping your muscles contract properly. Healthy sodium levels are essential because even small shifts can affect how you feel on a daily basis. When sodium is well balanced, your body maintains the right amount of fluid in your blood vessels and tissues, your nerves transmit signals efficiently, and your muscles, including your heart, contract in a coordinated way. Because sodium has such far reaching effects, your body works hard to keep it within a remarkably tight range. Imagine your bloodstream as a carefully calibrated solution. Sodium acts like the key ingredient that determines how concentrated that solution is, and your kidneys serve as the master regulators, constantly adjusting how much sodium they retain or flush out in your urine. Hormones such as aldosterone and antidiuretic hormone fine-tune this process, responding to signals from your brain and adrenal glands. When you drink more water, your kidneys pass the excess in urine; when you are dehydrated, they conserve water. This elegant feedback loop runs around the clock without you ever having to think about it. Your sodium level can be influenced by how much fluid you drink, your dietary salt intake, and how much you sweat, particularly during exercise or hot weather. Certain medications, especially diuretics and some blood pressure treatments, can also shift your reading. Prolonged vomiting, diarrhoea, or intensive endurance exercise may temporarily lower sodium, while significant dehydration can push it higher. If your sodium result falls outside the normal range, it is a good idea to discuss it with your GP. They can assess whether it reflects something straightforward like hydration status or whether it warrants a closer look at your kidney function or hormone balance.

Chloride is an essential electrolyte that works hand in hand with sodium to keep your body's fluid balance and acid-base chemistry running smoothly. This blood test measures the concentration of chloride ions in your bloodstream, giving your healthcare team a window into how well your body is maintaining its internal environment. Balanced chloride levels matter because this electrolyte is a key player in several vital processes. It helps your kidneys manage the volume and composition of your body fluids, contributes to the production of stomach acid needed for digestion, and plays a supporting role in nerve impulse transmission. When chloride and sodium are both in the expected range, it suggests your body's chemistry is well regulated. Think of chloride as sodium's quiet partner. While sodium often gets the spotlight, chloride travels alongside it through your bloodstream and across cell membranes, helping to balance the electrical charge on either side. Together, they create the conditions your cells need to function. Chloride also helps maintain your blood's pH, the delicate acid-base balance that your body guards carefully. Your kidneys are the main regulators here, deciding how much chloride to keep and how much to release, adjusting constantly based on what your body needs at any given moment. Because chloride and sodium are so closely linked, many of the same factors affect both. Dehydration tends to push chloride levels up, while drinking large amounts of fluid can dilute them. Prolonged vomiting can lower chloride because stomach acid contains a significant amount of it. Certain medications, particularly diuretics and some antacids, may also influence your reading. Heavy sweating during exercise can temporarily reduce chloride levels as well. If your chloride result is outside the expected range, it is worth mentioning to your GP. They will often look at it alongside your sodium, kidney function markers, and hydration status to understand what might be going on and whether any follow-up is needed.

Creatinine is a waste product from muscle metabolism that the kidneys remove from blood. Its concentration is used to estimate kidney filtration as eGFR. Muscle mass, recent meat intake, creatine supplements and some medicines can affect creatinine independently of filtration. A change in creatinine needs clinical context rather than an assumption that it is either harmless or caused by kidney disease. Cystatin C can provide complementary information when creatinine is difficult to interpret. An equation using both markers generally estimates filtration more accurately than either alone, although each marker has limitations. A clinician can consider trends, urine tests and other findings alongside the estimate.

BUN (Blood Urea Nitrogen) measures exactly the same substance as urea. It is simply a different way of reporting the same molecule. The urea molecule contains nitrogen, and BUN quantifies just that nitrogen fraction rather than the whole molecule. US laboratories and North American medical literature tend to report BUN (in mg/dL), while UK and European labs report total urea (in mmol/L). Seeing BUN on your results, or hearing it used by an American source, does not mean a new test has been done. It is the same measurement in a different notation. To convert, BUN in mg/dL roughly equals urea in mmol/L multiplied by 2.8. Why do two conventions exist at all? Historically, early analytical methods measured the nitrogen content of blood protein products directly, which is where the BUN nomenclature comes from. Over time, newer methods allowed direct measurement of the whole urea molecule, and UK labs moved to reporting it that way. The underlying biology is identical: your liver converts ammonia, produced during protein breakdown, into urea, and your kidneys filter urea out of the blood and excrete it in urine. Whether called BUN or urea, this marker reflects how well your liver produces it and how efficiently your kidneys clear it. Everything that raises or lowers urea raises or lowers BUN in the same way. A high BUN is commonly caused by dehydration (which concentrates waste products), a high-protein diet, intense exercise, gastrointestinal bleeding, heart failure, or reduced kidney filtration. A low BUN can reflect a low-protein or plant-based diet, overhydration, pregnancy, or, uncommonly, significant liver disease where the liver is no longer producing urea efficiently. Certain medications, including corticosteroids and some diuretics, can also shift the result. If your BUN is outside the reference range, your GP will interpret it alongside creatinine and eGFR to decide whether the pattern points to a simple cause (dehydration, diet) or warrants a closer look at kidney function. In isolation, a raised BUN often reflects dehydration or a high protein intake.

Urea is a waste product that your liver produces every time you break down protein from food. Once made, it travels in your bloodstream to the kidneys, where it is filtered out and excreted in your urine. Measuring urea in the blood therefore gives a useful window into two things at once: how your liver is processing protein and how well your kidneys are clearing waste. It is a long-established marker on almost every routine blood panel and is usually interpreted alongside creatinine and eGFR when assessing kidney function. Think of urea as part of a relay race. Proteins from your diet are broken down into amino acids, which produce ammonia as a by-product. Ammonia is toxic to the brain and nervous system, so your liver converts it into urea through a series of reactions called the urea cycle. Urea is much less harmful and dissolves easily in blood, which makes it easy for your kidneys to filter and remove. A healthy urea level suggests that both handoffs, liver production and kidney clearance, are running smoothly. In the UK urea is reported directly in mmol/L, while in the US the same measurement appears as BUN (Blood Urea Nitrogen) in mg/dL. Your urea result is more sensitive to lifestyle than most kidney markers. A high-protein diet (lots of meat, fish, eggs, or protein shakes) naturally pushes urea higher, as does dehydration, which concentrates everything in the blood. It can also rise after a gastrointestinal bleed (as the blood itself acts as a protein load), during periods of illness or rapid weight loss, and with certain steroid medications. A low urea usually points the other way: a lower-protein or plant-based diet, overhydration, pregnancy, or, less commonly, significant liver disease where the liver cannot produce urea efficiently. If your urea is out of range, the most common explanations are dietary or hydration-related rather than kidney disease. Your GP will look at urea alongside your creatinine, eGFR, and often your sodium and haematocrit to decide whether the picture points to dehydration, diet, or a genuine kidney or liver concern. They may suggest a repeat test after improving fluid intake.

Estimated glomerular filtration rate (eGFR) describes how quickly the kidneys are estimated to filter blood. It is calculated from creatinine, cystatin C or both, together with factors such as age and sex. It is an estimate rather than a direct measurement. Creatinine-based estimates can be less reliable when muscle mass or other influences on creatinine differ from the assumptions of the equation. Cystatin C is less influenced by muscle mass but has its own limitations. Combining the two markers generally improves accuracy; clinical circumstances can favour another approach. A single eGFR does not give the whole picture of kidney health. Clinicians consider changes over time, urine albumin and other evidence. An unexpected or persistently reduced estimate should be assessed in that wider context.

Cystatin C is a protein produced by nucleated cells and filtered from blood by the kidneys. Its blood concentration can be used to estimate filtration. It is less influenced by muscle mass than creatinine, making it useful when creatinine is difficult to interpret. Cystatin C is not a perfect or universally superior marker. Thyroid dysfunction, steroid medicines and other non-kidney factors can influence it. A change should therefore be interpreted alongside clinical context. An equation combining creatinine and cystatin C generally gives a more accurate estimate than either marker alone. A clinician can decide which estimate is appropriate and consider it alongside urine tests and changes over time.