3.1 Diagnostic & Laboratory Testing
Key Takeaways
- Withhold metformin before iodinated contrast studies and restart only after 48 hours once renal function is confirmed adequate, because contrast-induced nephropathy plus metformin can precipitate lactic acidosis
- A potassium of 6.0 mEq/L is a critical value: place the patient on a cardiac monitor and anticipate IV calcium gluconate first to stabilize the myocardium, then insulin with dextrose to shift potassium intracellularly
- A therapeutic INR for most warfarin indications is 2.0-3.0 (2.5-3.5 for mechanical mitral valves); an INR above 5 carries significant bleeding risk and may require vitamin K
- Digoxin toxicity is likely above 2.0 ng/mL, and risk climbs sharply when potassium is low — always check K+ before giving digoxin
- ABG interpretation follows a three-step read: pH for acidosis/alkalosis, PaCO2 for the respiratory component, HCO3 for the metabolic component, then check for compensation
Preparing Patients for Diagnostic Tests
Safe preparation is heavily tested because preparation errors cause real patient harm. Know the classic teaching points cold.
Fasting Requirements
- Fasting glucose and lipid panels: nothing by mouth (NPO) except water for 8-12 hours. Coffee with cream invalidates a fasting lipid panel.
- Procedural sedation: clear liquids up to 2 hours and light solids up to 6-8 hours before anesthesia, per standard fasting guidelines.
- Glucose tolerance test: fasting specimen first, then timed draws after the glucose load — do not let the patient eat or smoke between draws.
Contrast Media Precautions
Iodinated contrast (CT, cardiac catheterization, IV pyelogram) drives several high-yield rules:
- Allergy screening: ask specifically about prior reactions to iodinated contrast, not just "allergies." The traditional shellfish link is largely a myth, but the ANCC exam still expects you to document and report any prior contrast reaction. A patient with a known reaction is premedicated with a corticosteroid (prednisone) and an antihistamine (diphenhydramine).
- Metformin hold: metformin is withheld on the day of the contrast study and for 48 hours after, restarted only after renal function is reconfirmed. If contrast injures the kidney, metformin accumulates and can cause fatal lactic acidosis.
- Renal protection: check baseline creatinine and estimated glomerular filtration rate (eGFR); hydrate before and after; monitor urine output (goal at least 30 mL/hr).
- Gadolinium (MRI): the concern in severe renal disease is nephrogenic systemic fibrosis — screen eGFR before MRI with contrast. Also confirm no implanted metal, pacemakers that are not MRI-conditional, or shrapnel.
Bowel Preparation
For colonoscopy and bowel surgery, the patient follows a clear-liquid diet and completes a prescribed bowel prep (polyethylene glycol solution or sodium phosphate agents). Teach the patient to finish the entire prep — poor prep means poor visualization and a repeated procedure. Clinical pearl: sodium phosphate preps are risky in renal failure and heart failure because of phosphate load and fluid shifts.
Critical (Panic) Values That Demand Immediate Action
A critical value is a result so far outside the reference range that it signals a life-threatening state. The nurse's duties: verify the result (was the sample drawn correctly? hemolyzed?), assess the patient, notify the provider promptly, document who was notified and when, and act on standing orders or protocols.
| Value | Critical Level | Why It Kills | Immediate Nursing Response |
|---|---|---|---|
| Potassium | >6.0 mEq/L | Ventricular dysrhythmias, peaked T waves, widened QRS | Cardiac monitor; anticipate IV calcium gluconate, then regular insulin + dextrose, sodium polystyrene sulfonate |
| Glucose | <50 mg/dL | Altered LOC, seizures, brain injury | Conscious: 15 g fast oral carbohydrate; unconscious: IV dextrose 50% or IM glucagon |
| Digoxin | >2.0 ng/mL | Toxicity: anorexia, nausea, visual halos, heart block | Hold the dose; check potassium (hypokalemia potentiates toxicity); anticipate digoxin immune Fab for severe toxicity |
| INR | >5 on warfarin | Spontaneous hemorrhage | Assess for bleeding; anticipate vitamin K (oral/IV) and fresh frozen plasma for active bleeding |
| Troponin | Any elevation | Myocardial injury/infarction | Continuous ECG, serial troponins, prepare for reperfusion pathway |
| Lactate | >2 mmol/L concerning, >4 critical | Tissue hypoperfusion, sepsis | Draw with blood cultures, anticipate sepsis bundle: cultures, broad-spectrum antibiotics, 30 mL/kg crystalloid |
| Platelets | <50,000/mm3 | Spontaneous bleeding risk | Bleeding precautions: no IM injections, soft toothbrush, electric razor, avoid rectal temps |
| Hemoglobin | <7 g/dL | Inadequate oxygen delivery | Assess for active bleeding; prepare for transfusion per policy |
Trap warning: a hemolyzed specimen falsely elevates potassium. If the patient looks well and the draw was difficult, question the result and redraw before treating — but never delay assessment while you do it.
Medications That Alter Results
- Loop and thiazide diuretics: hypokalemia, hyponatremia, hyperglycemia; furosemide also elevates uric acid.
- Corticosteroids: hyperglycemia, leukocytosis (demargination, not infection), masked fever.
- Heparin: prolongs aPTT; warfarin prolongs PT/INR. Drawing coagulation studies from a heparin-flushed line falsely elevates aPTT — draw from a fresh stick or discard adequately.
- ACE inhibitors/ARBs and potassium-sparing diuretics: hyperkalemia.
- Biotin supplements: can falsely lower troponin and distort thyroid assays on some platforms — ask about supplements.
- NSAIDs and aminoglycosides: elevate creatinine and BUN.
Core Panels: Normal Ranges and Rapid Interpretation
Complete Blood Count (CBC)
| Component | Normal Range | Clinical Meaning |
|---|---|---|
| WBC | 4,500-11,000/mm3 | High: infection, steroids; Low: immunosuppression |
| RBC | 4.2-5.4 million/mm3 (M), 3.6-5.0 (F) | Oxygen-carrying capacity |
| Hemoglobin | 14-18 g/dL (M), 12-16 (F) | Anemia below ~12; transfusion threshold often 7-8 |
| Hematocrit | 42-52% (M), 37-47% (F) | Roughly 3x hemoglobin |
| Platelets | 150,000-400,000/mm3 | <50,000 bleeding risk; <20,000 spontaneous bleed risk |
Basic Metabolic Panel (BMP)
| Component | Normal Range |
|---|---|
| Sodium | 135-145 mEq/L |
| Potassium | 3.5-5.0 mEq/L |
| Chloride | 98-106 mEq/L |
| CO2 (bicarbonate) | 22-26 mEq/L |
| BUN | 10-20 mg/dL |
| Creatinine | 0.6-1.2 mg/dL |
| Glucose (fasting) | 70-100 mg/dL |
| Calcium (total) | 8.5-10.5 mg/dL |
A BUN-to-creatinine ratio above 20:1 with both elevated suggests dehydration or prerenal azotemia; creatinine is the more specific marker of renal function.
Coagulation Studies
| Test | Normal | Therapeutic on Anticoagulant |
|---|---|---|
| PT | 11-12.5 seconds | Reported as INR |
| INR | 0.8-1.1 | 2.0-3.0 (2.5-3.5 mechanical mitral valve) |
| aPTT | 25-35 seconds | 1.5-2.5x control on heparin |
Remember the pairing: heparin-aPTT, warfarin-PT/INR. Reversal pairing: heparin-protamine sulfate, warfarin-vitamin K (and FFP or PCC for major bleeding).
Arterial Blood Gas (ABG) Basics
| Component | Normal |
|---|---|
| pH | 7.35-7.45 |
| PaCO2 | 35-45 mm Hg |
| HCO3 | 22-26 mEq/L |
| PaO2 | 80-100 mm Hg |
| SaO2 | 95-100% |
Three-step interpretation: (1) Look at pH — below 7.35 is acidosis, above 7.45 is alkalosis. (2) Look at PaCO2 — it moves opposite the pH in respiratory disorders (CO2 is an acid). (3) Look at HCO3 — it moves with the pH in metabolic disorders. If the component that matches the pH abnormality is the cause, the other value moving in the opposite direction indicates compensation. Classic med-surg example: a COPD patient with pH 7.34, PaCO2 58, HCO3 30 has respiratory acidosis with metabolic compensation — and giving high-flow oxygen can knock out the hypoxic drive in a chronic CO2 retainer.
The Nurse's Response to Abnormal Values
- Assess the patient first, not the paper. A lab value in an asymptomatic patient is verified and reported; the same value with symptoms triggers immediate intervention.
- Verify: check for hemolysis, wrong tube, contamination from an IV line, and timing relative to doses (peak/trough).
- Notify and document: critical values require prompt provider notification with read-back per policy; document time, person notified, and response.
- Act on protocols: hypoglycemia protocols, potassium replacement protocols, sepsis bundles, and transfusion thresholds are standing interventions nurses initiate independently.
- Reassess: every intervention for an abnormal value demands a scheduled reassessment — repeat glucose 15 minutes after treatment, recheck potassium after replacement, repeat lactate to trend sepsis resuscitation.
ANCC scenario items frequently present a critical value plus a distractor task (documentation, a routine med pass) — the correct answer is almost always assess the patient or perform the physiological intervention first.
A patient with type 2 diabetes taking metformin is scheduled for a CT scan with IV iodinated contrast. Which instruction is most appropriate?
The laboratory calls with a critical potassium of 6.2 mEq/L for a patient with chronic kidney disease. The ECG shows peaked T waves. Which action should the nurse anticipate first?
A patient on warfarin therapy for atrial fibrillation has an INR of 5.8 with no signs of active bleeding. Which outcome does this result place the patient at greatest risk for, and what should the nurse anticipate?