3 Electrolytes, Renal Function, and Fluid Balance

Interpret laboratory results in context

Laboratory reference intervals vary by laboratory, method, age, and clinical context. Use the range printed on the patient’s report, compare results with prior values, and assess the patient rather than interpreting a number alone. An out-of-range result is not automatically an emergency, but critical results or new symptoms require prompt action.

Approximate common adult reference values include:

• Sodium: $135\text{–}145\ \text{mEq/L}$; reflects water balance and neurologic function.

• Potassium: $3.5\text{–}5.0\ \text{mEq/L}$; important for cardiac conduction and muscle function.

• Chloride: $98\text{–}106\ \text{mEq/L}$; relates to fluid and acid–base balance.

• Carbon dioxide or bicarbonate: $22\text{–}29\ \text{mEq/L}$; relates to acid–base status.

• Total calcium: $8.5\text{–}10.5\ \text{mg/dL}$; important for neuromuscular and cardiac function. Interpret with albumin or ionized calcium when indicated.

• Magnesium: $1.7\text{–}2.2\ \text{mg/dL}$; important for neuromuscular and cardiac function.

• Phosphate: $2.5\text{–}4.5\ \text{mg/dL}$; relates to energy metabolism and bone-mineral balance.

• BUN: $6\text{–}20\ \text{mg/dL}$; relates to waste clearance, but is also affected by hydration, diet, and other factors.

• Creatinine: about $0.6\text{–}1.3\ \text{mg/dL}$; relates to waste clearance and should be interpreted in the context of baseline and muscle mass.

• eGFR: usually at least 90\ \text{mL/min/1.73 m^2} in healthy young adults; interpret with age and urine albumin.

• UACR: at most $30\ \text{mg/g}$; higher results may indicate kidney damage.

These are study-guide approximations, not universal cutoffs. For example, creatinine and electrolyte reference intervals differ among laboratories.

Sodium and neurologic risk

Sodium concentration reflects the balance of water relative to sodium; it does not simply measure total-body sodium.

Hyponatremia, below $135\ \text{mEq/L}$, can cause nausea, headache, confusion, lethargy, seizures, or coma, especially when it develops quickly. Possible causes include excess water relative to sodium, diuretics, gastrointestinal losses, heart or kidney disease, and SIADH. Hypernatremia, above $145\ \text{mEq/L}$, usually reflects water deficit or impaired access to water; thirst, restlessness, weakness, confusion, and seizures may occur.

For new confusion, seizure, reduced level of consciousness, or a critical sodium result, promptly assess airway, breathing, circulation, vital signs, and neurologic status. Initiate safety precautions, notify the responsible clinician, and follow emergency protocols. Do not independently give free water or hypertonic saline. Sodium correction must be prescribed and monitored carefully: overly rapid correction of chronic hyponatremia can cause serious neurologic injury.

Potassium and cardiac risk

Hypokalemia, below $3.5\ \text{mEq/L}$, may cause weakness, cramps, constipation, or ileus. More severe deficiency can cause ECG changes and dangerous arrhythmias, with greater risk in people with heart disease or taking digoxin. Hyperkalemia, above $5.0\ \text{mEq/L}$, may have no symptoms until cardiac toxicity develops. Weakness and ECG changes such as peaked T waves or widened QRS may occur; kidney impairment and some medications increase risk.

A critical potassium result, palpitations, weakness, or ECG changes warrant prompt escalation and cardiac monitoring according to protocol. Check the result, trend, renal function, and medication history. Hemolysis can falsely elevate potassium; if suspected, arrange a repeat sample according to policy, but do not delay escalation when the patient is symptomatic or the ECG is abnormal.

Potassium replacement, especially intravenous replacement, requires an order and prescribed rate. Never give potassium by IV push.

Calcium, magnesium, and phosphate

Low calcium can cause tingling, cramps, tetany, or seizures; severe cases may prolong the QT interval. High calcium may cause constipation, thirst, frequent urination, weakness, or confusion.

Low magnesium may cause tremor, cramps, hyperreflexia, seizures, or arrhythmias. Magnesium deficiency can make low potassium or calcium difficult to correct. High magnesium, particularly with kidney failure or magnesium-containing medicines, can cause diminished reflexes, hypotension, bradycardia, and respiratory depression.

Low phosphate may cause weakness; severe deficiency can impair muscle and respiratory function or alter mental status. High phosphate is common in advanced kidney disease and can contribute to low calcium and bone-mineral problems.

Promptly escalate severe symptoms, seizures, arrhythmias, respiratory changes, or critical results. Assess mental status, muscle strength, reflexes, ECG, kidney function, and related electrolyte results as appropriate. Give replacement or other treatment only as ordered; renal impairment can change both the risk and the safe replacement plan.

Assess renal function and possible AKI

Creatinine rises when filtration falls, but a single value can be misleading because muscle mass and other factors affect it. Compare creatinine with the patient’s baseline and trend over time. eGFR estimates filtration from blood tests. A persistent eGFR below 60\ \text{mL/min/1.73 m^2} or persistent evidence of kidney damage can indicate chronic kidney disease; UACR above $30\ \text{mg/g}$ is abnormal and may signal kidney damage. These findings require clinical interpretation, and CKD classification requires persistence over time.

BUN may rise with reduced kidney clearance, but it may also rise with dehydration, high protein intake, or gastrointestinal bleeding. It is not a stand-alone measure of kidney function.

Acute kidney injury may be suggested by any of these findings: creatinine rises at least $0.3\ \text{mg/dL}$ within $48$ hours; creatinine rises to at least $1.5$ times baseline within $7$ days; or urine output is below $0.5\ \text{mL/kg/hour}$ for at least $6$ hours.

Report new creatinine increases, falling eGFR, or reduced urine output promptly. Confirm accurate intake and output, review trends and relevant medications, assess volume status, and check for possible obstruction or other changes. Do not assume low urine output is dehydration or give fluids without considering signs of overload and the care plan.

Assess fluid balance

Use several findings together to assess fluid balance: measured intake and output, urine-output trend, daily weight, vital signs, edema, lung sounds, mucous membranes, mental status, and laboratory results when ordered. A rapid weight change can help reveal fluid gain or loss; approximately $1\ \text{kg}$ of body-weight change corresponds to $1\ \text{L}$ of fluid, although other factors can affect weight.

Possible volume depletion findings include thirst, dry mucosa, tachycardia, orthostatic hypotension, concentrated urine, dizziness, and reduced output. Severe depletion can progress to confusion, poor perfusion, and shock.

Possible fluid overload findings include increasing weight, edema, elevated blood pressure, jugular venous distension, crackles, and shortness of breath. New or worsening breathing difficulty is urgent.

Recheck concerning observations, measure and document output accurately, obtain or verify weight using consistent conditions, assess respiratory and circulatory status, and promptly report deterioration. Follow prescribed fluid limits or replacement orders; do not independently change them.

Apply findings to urgent decisions

In a patient with kidney impairment, potassium of $6.4\ \text{mEq/L}$ with new peaked T waves is an urgent cardiac risk. Promptly notify the responsible clinician or activate the appropriate emergency response, maintain cardiac monitoring, and follow facility protocol; do not wait for symptoms to worsen.

If sodium falls to $121\ \text{mEq/L}$ and the patient becomes confused, assess immediate safety, airway, and neurologic status, institute precautions, and promptly escalate the change. Sodium treatment and monitoring require orders because overly rapid correction can cause neurologic harm.

A creatinine increase from $0.9$ to $1.3\ \text{mg/dL}$ over $24$ hours is a rise of $0.4\ \text{mg/dL}$, meeting a common AKI criterion. For a $60$-kg adult with urine output of $20\ \text{mL/hour}$ for $6$ hours, output is about $0.33\ \text{mL/kg/hour}$, below the AKI urine-output threshold. Promptly report the findings, verify measurements, and assess the patient and fluid status.

If magnesium is low and potassium remains low despite prescribed replacement, magnesium deficiency may contribute to persistent hypokalemia. Report the paired abnormalities and follow orders for further evaluation and treatment.

For a patient with heart failure who has new crackles, worsening shortness of breath, and rapid weight gain, promptly assess respiratory status and escalate the change because these findings may indicate worsening fluid overload. Do not give additional fluids without an order and reassessment.

Integrate the findings

Interpret laboratory results using the local reference interval, the patient’s baseline, trends, symptoms, and overall fluid status. Sodium abnormalities primarily threaten neurologic function; potassium, calcium, and magnesium abnormalities can threaten cardiac function. Rising creatinine or reduced urine output may indicate AKI. For critical results or clinical deterioration, assess promptly, monitor appropriately, and escalate according to orders and facility protocol.

• Acute kidney injury (AKI)