Summary
Highlights
NPNs are important analytes for assessing kidney function. While urea is the primary NPN, uric acid and ammonia are also discussed, noting their primary relevance to gout and liver function, respectively, though they all pass through the kidneys. BUN accounts for 45% of NPNs, amino acids 20%, uric acid and creatinine 20%, creatine 5%, and ammonia 0.2%.
BUN is a major end product of dietary protein catabolism, with nitrogen sourced from protein. The normal BUN to creatinine ratio is 10:1 to 20:1. BUN is often the first metabolite to elevate in kidney disease and is easily removed by dialysis. It is influenced by nitrogen intake and hydration, as well as protein diet. To calculate urea from BUN, multiply BUN by 2.14.
Various methods are used to detect urea, including Nessler's reaction, Berthelot reaction (using urease to hydrolyze urea into ammonia and carbonate), glutamate dehydrogenase coupled enzymatic reaction (measuring NAD at 340 nm), conductometric methods (Eurograph or Eurase strat strip), and direct methods like DAM (Diacetylmonoxime) or Feron method. The reference method for urea is isotope dilution mass spectrophotometry.
Elevated BUN can indicate chronic renal diseases, stress, burns, high protein diet, poor nutrition, or impaired absorption. Uremia, characterized by elevated urea, can lead to hyperkalemia and acidemia. Azotemia, an increase in NPNs, is classified as pre-renal (reduced renal blood flow, e.g., hemorrhage, dehydration, congestive heart failure), renal (kidney disease affecting glomerular filtration rate), or post-renal (obstruction in urine flow).
Creatinine is a metabolite of creatine phosphate, powering muscles. Unlike BUN, it's not affected by protein diet and is stable in serum and urine. Creatinine is inversely related to glomerular filtration rate (GFR) and is a common monitor for renal function. It's also used to evaluate the completeness of 24-hour urine collections. It's affected by muscle mass, muscular diseases, and hyperthyroidism.
The Jaffé reaction is a common method, where creatinine reacts with alkaline picrate to form a yellow-orange tautomer read at 510 nm. Kinetic Jaffé is rapid, inexpensive, and has fewer interferences. Modifications like Jaffé with absorbent (Lloyd's or Fuller's Earth method) offer increased sensitivity and specificity by removing interferences, though it is time-consuming. Coupled enzyme reactions are also used.
The BUN to creatinine ratio helps pinpoint the source of kidney dysfunction. A low ratio (<10:1) can indicate a low protein diet, acute tubular necrosis, or hepatic disease. A high BUN and creatinine with normal creatinine typically points to pre-renal azotemia, suggesting an issue outside the kidneys like dehydration or congestive heart failure.
Uric acid is a breakdown product of nucleic acids. Excess uric acid can deposit in joints and tissues, leading to inflammation and gout. While reabsorbed in the proximal tubules, it's not a highly reliable test for kidney function itself. Increased uric acid can be due to high intake of purine-rich foods, increased nucleic acid metabolism, or conditions like leukemia, lymphoma, and Lesch-Nyhan syndrome.
Redox reactions (using phosphotungstic acid) and enzymatic methods (using uricase to convert uric acid to allantoin) are used for determination. Enzymatic methods are considered simpler and more specific. Heparinized plasma or serum is preferred, and samples should be separated immediately. Fasting is unnecessary. For urine, an alkaline pH is required, and EDTA/fluoride should be avoided as anticoagulants.
Ammonia results from the catabolism of amino acids. Its levels are affected by protein diet and bacterial metabolism in the intestine. Ammonia is a reliable indicator for hepatic failure and Reye's syndrome, a rare genetic disease affecting children's brain and liver, often after a viral infection. High ammonia levels are used to assess the prognosis of Reye's syndrome.
Methods for ammonia determination include ion-selective electrodes (ISE), spectrophotometric methods, and glutamate dehydrogenase. Arterial blood samples are preferred and must be transported on ice immediately. Heparinized tubes are ideal. Special care must be taken to avoid environmental contamination from tobacco smoke or ammonia in glassware, reagents, or water.