Blood Work for an 18-Year-Old
For a healthy 18-year-old without specific symptoms or risk factors, routine blood work is not universally recommended by major guidelines; however, if blood work is being performed, a complete blood count (CBC) with differential and a comprehensive metabolic panel form the foundation of initial laboratory assessment.
Initial Laboratory Testing
The specific tests ordered should be guided by clinical context, symptoms, and risk factors rather than age alone. When blood work is indicated for an 18-year-old, the following approach is recommended:
Core Laboratory Tests
Complete Blood Count (CBC) with Differential: This evaluates red blood cells, white blood cells, hemoglobin, hematocrit, and platelets, providing information about oxygen-carrying capacity, immune function, and potential for anemia, infection, or blood disorders 1, 2.
Comprehensive Metabolic Panel: Should include serum electrolytes (sodium, potassium, chloride, bicarbonate, calcium, magnesium), blood urea nitrogen, serum creatinine, liver function tests, and fasting blood glucose 3.
Anemia Screening (If Indicated)
Hemoglobin thresholds for anemia diagnosis in individuals ≥18 years are: <135 g/L in males and <120 g/L in females 3.
These gender-specific values are based on the 5th percentile from NHANES III data and may not apply to pregnant or menstruating women, individuals living at high altitude, smokers, non-Caucasian populations, or those with chronic lung disease or hemoglobinopathy 3.
Blood Pressure Screening
Blood pressure should be measured at every routine visit starting at age 18 years 3.
If blood pressure is <120/80 mmHg, recheck at least every 2 years; if 120-139/80-89 mmHg, recheck annually 3.
Condition-Specific Testing
For Patients with Diabetes (Type 1)
If the 18-year-old has type 1 diabetes, additional testing is required:
Lipid profile should be performed soon after diagnosis once glycemia has improved, with repeat testing every 3 years if LDL cholesterol is <100 mg/dL 3.
Thyroid function: Measure thyroid-stimulating hormone at diagnosis when clinically stable, then recheck every 1-2 years 3.
Celiac screening: Measure IgA tissue transglutaminase antibodies with documentation of normal total serum IgA levels soon after diabetes diagnosis 3.
For Patients with Suspected Heart Failure
If cardiac symptoms are present:
- Initial laboratory evaluation should include CBC, urinalysis, serum electrolytes (including calcium and magnesium), blood urea nitrogen, serum creatinine, fasting blood glucose, glycohemoglobin, lipid profile, liver function tests, and thyroid-stimulating hormone 3.
Important Clinical Caveats
When NOT to Order Routine Blood Work
Preoperative screening: For healthy young patients undergoing minor procedures like adenoidectomy, routine coagulation tests and blood work have minimal clinical benefit and should only be performed if there is an unusual bleeding history, family bleeding history, or clinical indications for anemia 4.
Blood tests performed routinely without clinical indication rarely change management and can lead to unnecessary follow-up testing 4.
Key Pitfalls to Avoid
Do not use adult reference ranges indiscriminately: Anemia definitions may not apply to menstruating women, smokers, those at high altitude, or non-Caucasian populations 3.
Avoid ordering tests without clinical context: The role of blood testing is most valuable when guided by specific chronic disease risk factors, symptoms, or family history rather than age-based screening alone 5.
Document clinical reasoning: Always document why specific tests are being ordered, as this improves adherence to evidence-based practices and reduces unnecessary testing 5.