4.3 Patient History, Contraindications, and Pregnancy Screening

4.3 Patient History, Contraindications, and Pregnancy Screening

High-quality dual-energy x-ray absorptiometry begins well before x-ray exposure. Systematic patient verification, medical history acquisition, contraindication screening, and pre-scan attire preparation are critical technologist responsibilities. Errors during intake compromise patient safety, distort attenuation calculations, and generate misleading clinical interpretations.

Pre-Scan Patient History Verification and Intake Protocol

Clinical standards mandate verifying patient identity using two unique identifiers, typically full legal name and date of birth. Beyond identity confirmation, several demographic parameters directly govern software calculations:

1. Age and Date of Birth: Software algorithms calculate age-matched Z-scores and FRAX probabilities directly from date of birth. Entry errors distort reference calculations.
2. Sex Assigned at Birth: Peak adult bone mass and reference distribution curves differ significantly between biological sexes. Normative databases compare patients against sex-matched young-normal reference cohorts. For transgender patients, clinicians enter natal sex or sex concordant with hormone therapy exceeding five years.
3. Race and Ethnicity: Skeletal geometry and peak bone mass vary across ethnic backgrounds. Major DXA manufacturers provide databases for Caucasian, African American, Hispanic, and Asian populations to ensure valid standard deviation reference comparisons.

Direct Physical Measurement Standards: Height and Weight

Accurate height and weight measurements are essential. Facilities must never rely on self-reported values, as patients systematically overestimate height and underestimate weight:

• Scanner software utilizes BMI and tissue thickness to select acquisition modes (thin, standard, or thick). Erroneous data leads to incorrect tube current or scan speed selection, degrading image quality.
• Direct measurement identifies occult vertebral compression fractures.

Clinical Height Loss Thresholds

Vertebral compression fractures frequently develop asymptomatically without acute trauma. Two validated height loss thresholds mandate clinical investigation:

• Historical Height Loss > 1.5 inches (> 4.0 cm): Difference between measured current standing height and recalled peak young-adult height.
• Prospective Height Loss > 0.8 inches (> 2.0 cm): Documented height loss between consecutive clinical appointments.

When either threshold is crossed, the technologist must notify the interpreting physician and recommend Vertebral Fracture Assessment (VFA) or spinal radiographs to evaluate for silent vertebral collapse.

Structured Pre-Scan Intake Questionnaire

The pre-scan questionnaire identifies clinical risk factors, previous examinations, and technical scanning impediments:

Contraindications in Bone Densitometry

Central DXA is non-invasive, yet key contraindications exist:

• Pregnancy (Absolute Contraindication for Screening): Elective screening densitometry is strictly contraindicated during pregnancy to eliminate fetal ionizing radiation exposure. In female patients of reproductive potential (ages 12 to 55), technologists must apply the 10-day rule (scheduling examinations within the first 10 days following the onset of menses) or administer a pregnancy screening questionnaire. If missed menses or pregnancy uncertainty exists, a clinical pregnancy test must be completed prior to scanning.
• Positional Limitations: Patients unable to lie supine due to severe kyphosis, pain, or respiratory distress cannot undergo central spine and hip imaging. In such cases, the nondominant forearm serves as an alternative peripheral site.
• Weight Capacity: DXA tables have weight limits (typically 350 to 500 lbs / 159 to 227 kg). Exceeding limits damages drive motors and invalidates magnification calibration.

Interfering Substances and Radiopaque Artifacts

Exogenous radiopaque substances in the scan field distort beam attenuation, producing spurious BMD values:

1. Radiopaque Contrast Media (Barium & CT Contrast): Barium sulfate (Z = 56) and iodinated contrast (Z = 53) attenuate x-ray beams. Retained contrast overlying the lumbar spine or pelvis simulates hyperdense bone, falsely elevating measured BMD and masking osteoporosis. Mandatory wait time: 1 to 2 weeks (7 to 14 days) for gastrointestinal clearance. A scout scan can verify clearance if uncertain.
2. Nuclear Medicine Radiopharmaceuticals: Injected radionuclides (e.g., Technetium-99m, Iodine-131, PET tracers) emit gamma rays that strike DXA detectors, adding false counts and corrupting attenuation ratios. Mandatory wait time: 48 to 72 hours (or 10 physical half-lives) of the isotope.
3. Oral Calcium Supplements: Calcium tablets taken on exam day may remain undissolved in bowel overlying L1–L4, creating focal radiopacities that falsely elevate spine BMD. Patients must abstain from calcium supplements for 24 hours prior to scanning.

Patient Attire and Artifact Removal

Before scanning, patients must remove external metallic and radiopaque items: zippers, snaps, buttons, wire hooks, underwire bras, buckles, jewelry, body piercings between mid-chest and thighs, and metallic transdermal patches. Patients should change into a metal-free cotton gown or scrub pants without fasteners.