3.4 Specimen Collection, Point-of-Care Testing, and Diagnostic Preparation
Key Takeaways
- Blood cultures must be drawn before the first dose of antibiotic; two sets from two separate sites, each with the skin antiseptic allowed to dry fully, are required to distinguish true bacteraemia from skin contamination.
- Sputum for acid-fast bacilli is collected as early-morning deep-cough specimens on consecutive days, in a negative-pressure room or open air with the nurse wearing a fit-tested N95 respirator.
- A clean-catch midstream urine specimen must reach the laboratory within one hour or be refrigerated, because bacterial overgrowth at room temperature falsely elevates the colony count.
- When drawing blood from a limb with an intravenous infusion running, use the opposite arm; if that is impossible, stop the infusion for at least two minutes and draw distal to the cannula, and never draw above a running line.
- Warfarin monitoring uses the INR, unfractionated heparin uses the aPTT, and vancomycin and aminoglycosides use trough levels drawn immediately before the next scheduled dose.
3.4 Specimen Collection, Point-of-Care Testing, and Diagnostic Preparation
A specimen is a clinical decision in a tube. Collect it at the wrong time, from the wrong site, or into the wrong container, and the laboratory will produce a technically perfect result about a sample that no longer represents the patient. Examiners test this indirectly: the stem describes a collection, and the correct answer is the one that preserves the validity of the result or protects the patient.
Blood Specimens
Order of draw matters because additives carry over between tubes. The conventional sequence is blood culture bottles first, then the citrate (light blue) coagulation tube, then serum tubes, then heparin, then EDTA, then fluoride/oxalate. Drawing an EDTA tube before a chemistry tube, for example, carries potassium-EDTA across and produces a spuriously high potassium and a falsely low calcium.
Blood cultures are the highest-stakes specimen on a ward.
- Draw before the first antibiotic dose. Once antibiotics are circulating, the yield drops sharply and the organism may never be identified.
- Take two sets from two separate venepuncture sites. Growth in one bottle only, particularly of a skin organism such as coagulase-negative staphylococcus, usually means contamination; growth in both sets supports true bacteraemia.
- Disinfect the skin and allow the antiseptic to dry completely — the contact time is what kills organisms, and wiping it off early is the commonest cause of a contaminated culture.
- Disinfect the bottle tops too, and inoculate the aerobic and anaerobic bottles with the volume the manufacturer specifies; under-filling reduces sensitivity.
- If the patient has a central line and catheter-related infection is suspected, paired peripheral and line cultures are drawn to compare time to positivity.
Never draw above a running intravenous infusion. Diluted, fluid-contaminated blood produces a falsely low haemoglobin and a chemistry profile that mirrors the infusion. Use the opposite arm. If that is genuinely impossible, stop the infusion for at least two minutes, apply the tourniquet and draw distal to the cannula, and document what was done.
Haemolysis — the other common pre-analytical error — falsely raises potassium, LDH, AST, and magnesium. It is caused by an excessively tight or prolonged tourniquet, a needle that is too fine, vigorous plunger pull, or shaking rather than gently inverting the tube.
Therapeutic drug monitoring timing
| Drug / test | When to draw | Target or note |
|---|---|---|
| Warfarin (INR) | Any time; consistent daily timing | Typically 2.0–3.0, higher for mechanical valves |
| Unfractionated heparin (aPTT) | 6 hours after a rate change, then daily | 1.5–2.5 × control |
| Low-molecular-weight heparin | Routine monitoring not required | Anti-Xa only in renal impairment, pregnancy, extremes of weight |
| Vancomycin | Trough, immediately before the next dose | Nephrotoxicity and ototoxicity risk |
| Gentamicin / amikacin | Trough before dose; peak 30 min after infusion ends | Ototoxicity, nephrotoxicity |
| Digoxin | ≥ 6–12 hours after the dose | 0.5–2.0 ng/mL; interpret against serum potassium |
| Lithium | 12 hours after the evening dose | 0.6–1.2 mEq/L |
| Phenytoin | Trough before the dose | 10–20 mcg/mL |
Urine, Sputum, Stool, and Wound Specimens
Clean-catch midstream urine. Teach perineal cleaning front to back, void the first portion into the toilet to flush the distal urethra, then catch the midstream in a sterile container. Deliver to the laboratory within one hour, or refrigerate for up to 24 hours — bacteria multiply at room temperature and inflate the colony count into a false "infection". A 24-hour urine collection starts by discarding the first void and noting the time, then collecting every subsequent void including the final one at the 24-hour mark; a single discarded void invalidates the whole collection and it must restart.
Sputum. Collect early morning, when overnight secretions have pooled, after mouth rinsing with plain water (not antiseptic mouthwash, which kills the organisms). Ask for a deep cough from the lungs — saliva is not sputum, and the laboratory will reject it. For acid-fast bacilli, collect on consecutive days per local protocol, in a negative-pressure room or in open air, with the nurse wearing a fit-tested N95 respirator. For cytology, an early-morning specimen on three consecutive days is typical.
Stool. Use a clean dry container; urine and toilet water destroy parasites and dilute the specimen. For ova and parasites, deliver warm and promptly, because trophozoites disintegrate as the specimen cools. For occult blood testing, the patient may need to avoid red meat, certain vegetables, and vitamin C for several days beforehand, depending on the test used. For suspected Clostridioides difficile, send only unformed stool — testing formed stool detects colonisation rather than disease.
Wound swab. Clean the wound with normal saline first to remove surface exudate and colonising flora, then swab the viable tissue at the wound base using a rotating motion, not the slough or the surrounding skin. Swabbing pus alone frequently grows contaminants and misdirects antibiotic therapy.
Point-of-Care Testing
Point-of-care devices put laboratory decisions at the bedside, so the nurse owns their quality control.
- Capillary blood glucose: wash hands with soap and water and dry them; alcohol residue and fruit juice on the skin both distort the reading. Use the side of the fingertip, where there are fewer nerve endings. Discard the first drop. Cross-check against a laboratory sample when the value is extreme or inconsistent with the patient, because peripheral shutdown in shock makes capillary readings unreliable.
- Arterial blood gas: perform the modified Allen test before radial puncture to confirm ulnar collateral flow. Expel air bubbles, place the sample on ice if analysis is delayed, and apply firm pressure for at least 5 minutes, longer if the patient is anticoagulated.
- 12-lead ECG: V1 at the fourth intercostal space right sternal border, V2 fourth intercostal space left sternal border, V4 fifth intercostal space midclavicular line, V3 midway between V2 and V4, V5 fifth intercostal space anterior axillary line, V6 fifth intercostal space midaxillary line. Misplaced chest leads are a common cause of a spurious "anterior infarct" and an unnecessary escalation.
Preparing Patients for Diagnostic Procedures
| Procedure | Key nursing preparation | Key aftercare |
|---|---|---|
| Contrast CT | Check renal function and contrast allergy; confirm hydration; withhold metformin per local protocol | Encourage fluids; monitor creatinine; watch for delayed allergic reaction |
| MRI | Screen for pacemakers, implants, metal fragments, and cochlear implants; remove all metal | Routine; sedated patients need recovery monitoring |
| Upper GI endoscopy | Nil by mouth 6–8 hours; consent; remove dentures | Nil by mouth until the gag reflex returns; monitor for bleeding and perforation |
| Colonoscopy | Bowel preparation; clear fluids; consent | Monitor for perforation — pain, distension, fever, bleeding |
| Bronchoscopy | Nil by mouth; consent; baseline observations | Nil by mouth until gag reflex returns; watch for bleeding, bronchospasm |
| Liver biopsy | Check coagulation profile; group and save; consent | Lie on the right side to compress the site; strict bed rest; monitor for haemorrhage |
| Lumbar puncture | Empty bladder; positioning; consent | Lie flat as instructed; encourage fluids; assess for post-dural-puncture headache |
| Paracentesis | Empty bladder to avoid puncture; measure girth and weight | Monitor for hypotension after large-volume drainage |
The recurring safety theme across the endoscopic procedures is the gag reflex: topical pharyngeal anaesthesia abolishes airway protection, and the patient must remain nil by mouth until it demonstrably returns.
A patient with suspected sepsis has a temperature of 39.4 °C and a prescription for intravenous piperacillin-tazobactam stat plus blood cultures. The antibiotic has already arrived from pharmacy. In what order should the nurse act?
A nurse is asked to collect blood for a full blood count from a patient whose only available venous access is the left arm, where 0.9% sodium chloride is infusing through an antecubital cannula. What is the safest approach?
A patient receiving intravenous vancomycin for methicillin-resistant Staphylococcus aureus bacteraemia is prescribed a trough level. The next dose is scheduled for 08:00. When should the nurse collect the specimen?