3 Electrolyte, Renal, and Metabolic Studies

Interpreting laboratory results

Laboratory reference intervals can vary by method and patient. Use the ranges printed by the laboratory, and interpret results alongside symptoms, trends, medications, fluid status, and other tests rather than in isolation. A result marked critical requires prompt action according to facility policy.

Common adult reference ranges include:

• Sodium ($\mathrm{Na}^+$): $136\text{–}144\,\mathrm{mmol/L}$.

• Potassium ($\mathrm{K}^+$): $3.7\text{–}5.2\,\mathrm{mmol/L}$.

• Chloride ($\mathrm{Cl}^-$): $96\text{–}106\,\mathrm{mmol/L}$.

• Total CO₂, mostly bicarbonate: $23\text{–}29\,\mathrm{mmol/L}$.

• BUN: $6\text{–}20\,\mathrm{mg/dL}$.

• Creatinine: $0.8\text{–}1.2\,\mathrm{mg/dL}$.

• Glucose: $64\text{–}100\,\mathrm{mg/dL}$; fasting ranges differ by laboratory.

• Total calcium: $8.5\text{–}10.2\,\mathrm{mg/dL}$.

• Arterial pH: $7.35\text{–}7.45$.

• PaCO₂: $35\text{–}45\,\mathrm{mm\ Hg}$.

• HCO₃⁻: approximately $22\text{–}26\,\mathrm{mmol/L}$.

BMP ranges are representative examples, not universal cutoffs. The acid–base ranges are commonly used teaching values; confirm them against the laboratory’s reference interval.

Responding to electrolyte abnormalities

Electrolyte abnormalities may affect cardiac and neurologic function. Respond to symptoms and trends as well as the reported value, and follow facility protocols and prescribed orders.

High potassium

With chronic kidney disease, a potassium of $6.4\,\mathrm{mmol/L}$ and new muscle weakness should be treated as potentially urgent. Hyperkalemia can cause dangerous rhythm changes, and kidney disease can contribute to potassium retention. Assess the patient, promptly notify the responsible clinician, and initiate cardiac monitoring or obtain an ECG according to orders and facility protocol. Check for symptoms and review the result and specimen for possible hemolysis, but do not let a repeat draw delay escalation when the result is critical or the patient is symptomatic. Avoid potassium-containing medications or fluids unless specifically directed.

Low sodium

A sodium of $121\,\mathrm{mmol/L}$ with new confusion calls for prompt reporting of both the result and the mental-status change. Hyponatremia can cause neurologic complications when severe or symptomatic. Assess neurologic status and safety, then follow orders and facility protocol for monitoring and treatment. Do not attempt rapid correction independently; correction requires careful clinical management.

Low potassium

A patient taking a loop diuretic with potassium of $2.9\,\mathrm{mmol/L}$ and palpitations needs prompt assessment, clinician notification, and ECG or cardiac monitoring according to protocol. Review the medication and recent gastrointestinal losses with the care team. Hypokalemia may require potassium replacement, but give it only as prescribed, using the required route and monitoring precautions. Never give IV potassium by direct injection.

Assessing kidney function

A creatinine rise from $0.9\,\mathrm{mg/dL}$ to $1.6\,\mathrm{mg/dL}$ over two days, together with falling urine output, is concerning even if a single value might appear only moderately elevated. Report the trend and assess intake and output, vital signs, and volume status. Review recent medications and potential nephrotoxic exposures with the care team, and continue ordered monitoring of creatinine and urine output. Interpret Creatinine in context; eGFR and other findings also help assess kidney function.

Treating low blood glucose

Follow the facility’s hypoglycemia protocol. For a conscious patient who can safely swallow, give $15\,\mathrm{g}$ of fast-acting carbohydrate and recheck glucose in $15\,\mathrm{minutes}$. If glucose remains below $70\,\mathrm{mg/dL}$, repeat treatment according to protocol.

If the patient is unconscious or cannot swallow safely, give nothing by mouth. Call for immediate assistance and use prescribed glucagon or IV dextrose according to protocol. A conscious patient who received insulin and is shaky and diaphoretic, with a point-of-care glucose of $54\,\mathrm{mg/dL}$, is an example of a situation requiring this prompt protocol-based response.

Recognizing a possible metabolic emergency

Glucose of $390\,\mathrm{mg/dL}$ with vomiting, abdominal pain, and deep, rapid breathing calls for prompt escalation for urgent clinical assessment. Hyperglycemia with these symptoms may signal a serious metabolic emergency. Assess airway, breathing, circulation, vital signs, hydration, and mental status; check ketones and other tests as ordered. Do not assume that glucose alone establishes or rules out ketoacidosis.

Interpreting acid–base results

Begin with pH to identify acidemia or alkalemia. Then use PaCO₂ as the respiratory component and bicarbonate as the metabolic component. A blood-gas pattern identifies an acid–base disturbance but does not, by itself, establish its cause.

Respiratory pattern

For $\mathrm{pH}=7.28$, $\mathrm{PaCO}_2=58\,\mathrm{mm\ Hg}$, and $\mathrm{HCO}_3^-=26\,\mathrm{mmol/L}$, the low pH indicates acidemia. The elevated PaCO₂ points to primary Respiratory acidosis, while bicarbonate near the usual range suggests little or no metabolic compensation. Assess respiratory status and oxygenation, promptly report the result, and support ordered evaluation and treatment of the cause.

Metabolic pattern

For $\mathrm{pH}=7.29$, $\mathrm{HCO}_3^-=15\,\mathrm{mmol/L}$, and $\mathrm{PaCO}_2=30\,\mathrm{mm\ Hg}$, the low pH indicates acidemia and the low bicarbonate indicates primary Metabolic acidosis. The lower PaCO₂ is consistent with respiratory compensation. Assess the patient, promptly report the abnormal result, and review related findings and clinical context with the care team.

Putting findings into context

Recognize critical trends and symptoms, not just isolated values. Electrolyte abnormalities may affect neurologic and cardiac function; changing creatinine and urine output can signal kidney injury; and low glucose needs prompt treatment. Acid–base interpretation starts with pH, followed by PaCO₂ and bicarbonate. Escalate critical or symptomatic findings promptly and follow local protocols and prescribed orders.