What laboratory tests should be ordered for a patient with palpitations who is taking acetaminophen‑codeine (acetaminophen‑codeine) and could be pregnant?

Laboratory Tests for Pregnant Patient with Palpitations on Acetaminophen-Codeine

For a pregnant patient presenting with palpitations and documented tachycardia (pulse 119), you should immediately order: pregnancy test (serum β-hCG), complete blood count, comprehensive metabolic panel including electrolytes and renal function, thyroid function tests (TSH and free T4), troponin, and 12-lead ECG with continuous cardiac monitoring. 1, 2

Essential Immediate Laboratory Tests

Pregnancy Confirmation and Viability

• Serum β-hCG (quantitative pregnancy test) is mandatory in any woman of childbearing age with cardiac symptoms, as pregnancy status fundamentally changes management decisions and risk stratification 3
• If pregnancy is confirmed, obstetric ultrasound should be obtained urgently to document pregnancy viability and gestational age, as early pregnancy complications can manifest with cardiovascular symptoms 4

Cardiac-Specific Laboratory Tests

• Serum troponin to rule out acute coronary syndrome or myocardial injury, particularly given the chest discomfort and abnormal blood pressure (129/49) 3, 1
• 12-lead ECG immediately to confirm narrow complex tachycardia, evaluate for pre-excitation patterns (Wolff-Parkinson-White), and assess QT interval 1, 4
• Continuous cardiac monitoring to capture arrhythmia episodes and characterize the tachycardia type (supraventricular vs. ventricular) 3, 1

Metabolic and Electrolyte Panel

• Complete metabolic panel including sodium, potassium, magnesium, calcium, and glucose as electrolyte abnormalities are common precipitants of arrhythmias and can cause palpitations 3
• Serum creatinine and blood urea nitrogen to assess renal function, which affects drug clearance and is particularly important if antiarrhythmic therapy is needed 3

Thyroid Function Tests

• TSH and free T4 are essential, as hyperthyroidism is a critical differential diagnosis that can precipitate atrial fibrillation, atrial flutter, or sinus tachycardia during pregnancy 3, 1, 2
• The European Society of Cardiology specifically notes that thyroid storm can present with tachycardia, diaphoresis, and dyspnea—symptoms overlapping with this patient's presentation 2

Hematologic Assessment

• Complete blood count with differential to evaluate for anemia (which can cause compensatory tachycardia) and infection (which could indicate sepsis or other systemic illness) 3

Additional Tests Based on Clinical Context

Toxicology Considerations

• Urine toxicology screen should be considered given the patient is taking codeine, to assess for other substances that might contribute to tachycardia 3
• While acetaminophen pharmacokinetics are unchanged in pregnancy 5, serum acetaminophen level may be warranted if there is concern for overdose or hepatotoxicity

Structural Heart Disease Evaluation

• Echocardiography is recommended urgently to rule out structural heart disease, particularly peripartum cardiomyopathy, which can present with new-onset ventricular tachycardia during late pregnancy or postpartum 3, 1, 2, 4
• The European Society of Cardiology emphasizes that structural heart disease assessment is essential in any pregnant patient with documented arrhythmias 2

Critical Pitfalls to Avoid

• Never dismiss tachycardia as "normal pregnancy changes" without proper evaluation, as physiological sinus tachycardia in pregnancy typically does not exceed 100-110 bpm at rest, and this patient's pulse of 119 with symptoms warrants investigation 1, 4
• Do not delay cardioversion in hemodynamically unstable patients due to pregnancy concerns—maternal stability is essential for fetal survival 2, 4
• Avoid ordering only basic labs without cardiac-specific tests, as the documented tachycardia and abnormal blood pressure require immediate cardiac evaluation 1

Monitoring During Evaluation

• Continuous fetal monitoring is essential if pregnancy is confirmed, as maternal arrhythmias can cause fetal hypoperfusion 2, 4
• Serial vital signs and telemetry monitoring should continue throughout the evaluation to detect hemodynamic deterioration requiring immediate intervention 1, 2