Lecture on Diagnostic Testing

Diagnostic Lab Work and Differential Diagnosis

  • Laboratory work serves as a critical tool for healthcare providers to rule out or rule in various conditions based on a patient’s presenting symptoms.
  • The process of differential diagnosis involves identifying multiple potential causes for a symptom and systematically eliminating them through testing.
    • Example: Chest pain can signify a myocardial infarction (heart attack) or gastrointestinal issues like Gastroesophageal Reflux Disease (GERD) / acid reflux.
    • Diagnosis often mimics the approach shown in television dramas like House, where practitioners explore a list of about five possibilities before narrowing them down.
  • Abnormal and critical lab results must be reported to practitioners immediately, following specific facility policies, and all communications must be meticulously documented.

Contrast Dye and Diagnostic Imaging Considerations

  • Contrast dyes are utilized in CT scans (brain, lungs, etc.) and MRIs to highlight specific physiological structures or pathologies.
  • Assessments Prior to Contrast Administration:
    • Allergy Check: Patients must be screened for allergies to iodine or shellfish, as many contrast agents are iodine-based.
    • Alternative Agents: If a patient is allergic, they may receive non-iodine-based contrast.
    • Pre-medication: If a scan is essential and the patient is allergic, medications like Benadryl may be administered to mitigate reactions (e.g., tingling in the throat).
  • Renal Function Requirements:
    • Contrast agents, whether administered via IV or orally, must be excreted from the body by the kidneys.
    • Healthcare providers must review renal panels (BUN and Creatinine) before administering dye.
    • Pharmacokinetics and pharmacodynamics dictate that if renal function is impaired, specific medications may be used to assist the body in secreting the dye to prevent toxicity.
  • Post-Procedure Care (Oral Contrast):
    • Oral contrast is often used to visualize bowel obstructions (intestines will glow above a blockage but not below it).
    • Patients must maintain high fluid intake to ensure the contrast is excreted via stool.

Patient Consent and Autonomy

  • Autonomy: Patients have the right to refuse diagnostic tests, even those potentially life-saving. Coercion is prohibited.
  • Waivers and Advocacy: If a patient refuses, they must sign a waiver after being explained the risks of not having the test. The nurse’s role is to advocate for the patient’s right to make decisions for their own body.
  • The Consent Form:
    • The Practitioner’s Role: The surgeon, anesthesiologist, or practitioner performing the procedure (e.g., colonoscopy) is responsible for explaining the specific risks, benefits, and technical details. They are best positioned to answer clinical questions.
    • The Registered Nurse’s Role: The RN’s signature on a consent form indicates only that they witnessed the patient’s signature. It does not indicate the nurse explained the procedure or answered all technical questions.

Serum Laboratory Markers and Normal Ranges

  • Blood Urea Nitrogen (BUN):
    • A byproduct of protein metabolism (breakdown of protein).
    • Elevations can indicate impaired renal function, a high-protein diet, or dehydration.
    • High BUN levels often correlate with decreased filtration and low urine output.
  • Creatinine:
    • A waste product resulting from skeletal muscle breakdown.
    • It is a primary marker for assessing kidney function.
  • Blood Glucose and A1c:
    • Normal Serum Range: 70 to 110 mg/dL70\text{ to }110\,mg/dL.
    • Monitoring: Frequently monitored in diabetics.
    • Hemoglobin A1c (Glycosylated Hemoglobin): Reflects average blood sugar levels over a period of 2 to 32\text{ to }3 months. Values cannot be altered by short-term dietary changes (e.g., fasting for one week).
    • Fasting is usually required for random glucose serum tests.
  • Comprehensive Metabolic Panel (CMP):
    • Contains everything in a Basic Metabolic Panel (BMP).
    • Additional components: Liver enzymes (ALT/AST), protein, albumin, and bilirubin.

Clinical Procedures for Specimen Collection

  • Glucose Finger Stick (Accu-Chek):
    • 1. Ensure the finger is warm to promote blood flow (milking the finger).
    • 2. Use the ring finger or middle finger.
    • 3. Clean the site with alcohol.
    • 4. Puncture the side of the finger (avoid the pad due to high nerve density).
    • 5. Wipe away the first drop of blood with dry gauze.
    • 6. Collect the second drop onto the test strip.
  • 24-Hour Urine Specimen Collection:
    • Purpose: Collecting all urine produced over a full day to assess kidney function or solute levels.
    • Procedure:
      • The first void is flushed (discarded) to start the timer.
      • Every subsequent void for the next 24 hours is collected in a specific jug (often orange or red).
      • The jug must be kept on ice (in a basin) or in a dedicated specimen refrigerator.
      • The collection must be restarted from zero if any urine is accidentally flushed or if tissue/stool contaminates the sample.
    • Patient Education: Instruct the patient not to put toilet paper in the collection hat and to notify staff if they have a bowel movement so urine can be separated.
  • Urine Specific Gravity:
    • Reflects the concentration of solutes in the urine and indicates fluid status.
    • Normal Range: 1.005 to 1.0301.005\text{ to }1.030.
    • High Specific Gravity (> 1.030): Indicates dehydration (dark amber urine, high solute concentration).
    • Low Specific Gravity (< 1.005): Indicates fluid overload or overhydration (clear urine).
  • Stool Collection:
    • Testing: Used to detect parasites and eggs (Ova & Parasite/O&P), bacteria, viruses, and occult blood.
    • O&P samples must be sent to the lab while still warm.
    • Samples must not be contaminated with urine or toilet tissue.
  • Sputum Collection:
    • Best collected first thing in the morning.
    • The sample must be phlegm from the lungs, not just saliva/spit.
    • Culture and Sensitivity (C&S): Used to identify the specific bacteria and determine which antibiotics (gran-positive/gram-negative/broad-spectrum) the bacteria are susceptible to.

Gastrointestinal Bleeds and Stool Assessment

  • Upper GI Bleed:
    • Blood is broken down by hydrochloric acid in the stomach.
    • Result: Black, tarry, sticky stool (resembling molasses or tar).
    • Medical Term: Melena.
    • Note: Black stool (non-tarry) can also be caused by iron supplements or Pepto-Bismol.
  • Lower GI Bleed:
    • Blood is not digested.
    • Result: Frank blood (bright red blood) visible in or on the stool.
    • Common Causes: Hemorrhoids (internal or external varicose veins), colon cancer, or lower intestinal ulcers.
  • Occult Blood (Guaiac/Hemoccult Test):
    • Blood that is not visible to the naked eye.
    • Testing: A small smear of stool is placed on a Hemoccult card. A chemical reagent is applied; a color change (often to blue) indicates a positive result for blood.
  • Steatorrhea: Fatty, frothy, clay-colored stool that floats, indicating fat malabsorption. Often seen after gallbladder removal, as bile (which breaks down fat) is no longer stored and concentrated.

Diagnostic Imaging Modalities

  • KUB (Kidney, Ureters, Bladder): An X-ray used to visualize the size and position of these specific organs; not effective for soft tissue detail.
  • Ultrasound: Uses reflected sound waves to visualize soft tissues and organs (e.g., Echocardiogram is an ultrasound of the heart).
  • CT (Computed Tomography): Provides three-dimensional images of soft tissues with higher sensitivity than standard X-rays.
  • MRI (Magnetic Resonance Imaging):
    • Uses a strong magnetic field; provides superior contrast.
    • Safety: No metal objects (jewelry, clips, weld sparks in eyes) allowed due to the risk of injury or equipment damage.
    • Patient Experience: Very loud (clanging sounds) and can cause claustrophobia due to the narrow tube.
  • PET Scan (Positron Emission Tomography): Nuclear imaging that is the gold standard for identifying tumors and cancer tissue.
  • Mammogram: An X-ray of the breast soft tissue. Dense breasts may cause benign fibroadenomas to appear as lesions, necessitating follow-up with ultrasound or biopsy.
  • Angiography: Injection of contrast dye into vessels (radial or femoral access) to visualize blood flow and identify blockages via X-ray.
  • VQ Scan: A lung ventilation/perfusion scan used specifically to detect pulmonary embolisms (blood clots in the lungs).

Invasive Procedures and Nursing Care

  • Biopsy and Aspiration:
    • Needle Aspiration: Withdrawal of fluid and cells for testing.
    • Biopsy: Removing a small "pinch" or fragment of tissue/tumor using a specialized instrument that makes a "clicking" sound.
  • Lumbar Puncture (Spinal Tap):
    • Position: Patient in a fetal/crouched position to open the vertebrae.
    • Purpose: Accessing cerebrospinal fluid (CSF) to test for conditions like meningitis.
    • Post-Procedure: Patient must lie flat to prevent spinal headaches.
  • Paracentesis:
    • Draining fluid (ascites) from the abdominal cavity, often caused by liver failure or right-sided heart failure.
    • Risk: Fluid must be drained slowly. Removing too much too fast (e.g., liters) can lead to hypovolemic shock and a dangerous drop in blood pressure.
  • Thoracentesis: Removing fluid from the pleural space (around the lungs). The patient sits up and leans over a bedside table during the procedure.
  • Endoscopy (Bronchoscopy, EGD, Colonoscopy):
    • Involves conscious sedation (twilight sleep).
    • Safety: Patients must remain NPO (nothing by mouth) until their gag reflex returns to prevent aspiration.

Questions & Discussion

  • Question: Does an allergy to shellfish mean a patient absolutely cannot have a scan with contrast?
    • Response: No. If the scan is necessary to save a life, providers may administer Benadryl to manage the reaction, or they may use a contrast agent that is not iodine-bound.
  • Question: What does a nurse’s signature on a consent form mean?
    • Response: It indicates that the nurse witnessed the patient’s signature. It does not mean the nurse provided the technical explanation of the procedure; that is the practitioner's responsibility.
  • Question: Where is blood drawn from for ABGs?
    • Response: Arterial blood gases (ABGs) are drawn from an artery, unlike most other serum labs which are drawn from a vein.