1.6 Excretion

Excretion

Excretion is the final stage of a medication interaction within the body. The body has absorbed, distributed, and metabolized the medication molecules – now what does it do with the leftovers? Remaining parent drugs and metabolites in the bloodstream are often filtered by the kidney, where a portion undergoes reabsorption back into the bloodstream, and the remainder is excreted in the urine. The liver also excretes byproducts and waste into the bile. Another potential route of excretion is the lungs. For example, drugs like alcohol and the anesthetic gases are often eliminated by the lungs.^[1]

Routes of Excretion

Kidney

The most common route of excretion is through the kidneys. As the kidneys filter blood, the majority of drug byproducts and waste are excreted in the urine. The rate of excretion can be estimated by taking into consideration several client factors, including age, weight, biological sex, and kidney function. There are known sex differences in the three main renal functions of glomerular filtration, tubular secretion and tubular reabsorption. Renal clearance is generally higher in men than in women.^[2]

Kidney function is measured by lab values such as serum creatinine, glomerular filtration rate (GFR), and creatinine clearance. If a client’s kidney function is decreased, then their ability to excrete medication is affected, and drug dosages must be altered for safe administration.

Renal disorders, such as chronic kidney disease, can reduce kidney function and hinder drug excretion. As kidney function decreases with age, drug excretion becomes less efficient, and dosing adjustments may be needed. Other medical conditions that impact blood flow to the kidneys can also affect drug elimination. For example, heart failure can affect systemic blood flow to the kidney, resulting in decreased filtration and elimination of drugs.

Liver

As the liver filters blood, some drugs and their metabolites are actively transported by hepatocytes (liver cells) to bile. Bile moves through the bile ducts to the gallbladder and then on to the small intestine. During this process, some drugs may be partially absorbed by the intestine back into the bloodstream. Other drugs are biotransformed (metabolized) by intestinal bacteria and reabsorbed. Unabsorbed drugs and byproducts/metabolites are excreted in the feces.

If a client has decreased liver function, their ability to excrete medication is affected, and drug dosages must be adjusted. Lab studies used to evaluate liver function are called liver function tests and include measurement of alanine transaminase (ALT) and aspartate aminotransferase (AST) enzymes that the body releases in response to damage to or disease of the liver.

Conditions that cause decreased blood flow to the liver can also affect the metabolism and excretion of drugs. For example, conditions such as shock, hypovolemia, or hypotension cause decreased liver perfusion and may require adjustment of dosages of medication.

Other Routes to Consider

Sweat, tears, reproductive fluids (such as seminal fluid), and breast milk can also contain drugs and byproducts/metabolites of drugs. This can pose a toxic threat, such as the exposure of an infant to breast milk containing drugs or byproducts of drugs ingested by the mother. Therefore, nurses must refer to a drug reference and contact a health care provider with any concerns before administering medications to a mother who is breastfeeding.^[3]

Life Span Considerations

Neonate & Pediatrics

Neonates and children have immature kidneys with decreased glomerular filtration, resorption, and tubular secretion. As a result, they do not excrete medications as efficiently from the body. Dosing for most medications used to treat infants and pediatric clients is commonly based on weight in kilograms, and a smaller dose is usually prescribed. In addition, pediatric clients may have higher levels of free circulating medication than anticipated and may become toxic quickly. Therefore, it is vital for nurses to diligently recheck dosages before administering medications and closely monitor infants and children for early identification of adverse effects and drug toxicity.^[4]

Older Adult

Kidney and liver function often decrease with age, which can lead to decreased metabolism and excretion of medications. Subsequently, medication may have a prolonged half-life with a greater potential for toxicity due to elevated circulating drug levels. Some medications may be avoided or smaller doses recommended for older clients due to these factors, which is commonly referred to as “Start low and go slow.”^[5]

Putting It All Together

Safely administering medications to clients is a significant concern and requires team effort by pharmacists, prescribing health care providers, and nurses. In addition to the factors described in this chapter, there are many other considerations for safe medication administration that are further explained in the “Legal/Ethical” chapter.