Prescription for investigations
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Asking for Preferences
Asking for Preferences
Here is a practical clinical reference for writing and formatting a prescription/request for investigations (lab tests, imaging, procedures):
Prescription for Investigations - Clinical Reference
A request for investigations (also called a laboratory requisition, investigation request form, or diagnostic request) is a formal clinical document. Writing it correctly ensures the right test is performed, results are attributed to the right patient, and the clinician receives actionable data.
Mandatory Components
Every investigation request must include the following:
1. Patient Identification
- Full name (and alias if applicable)
- Date of birth (not just age)
- Gender (important for reference ranges)
- Hospital/MRN number (unique patient identifier)
- Ward / clinic / department
- Contact number (for critical value callbacks)
2. Requesting Clinician Details
- Full name and designation (e.g., Dr. J. Smith, SHO Medicine)
- Contact/bleep number - labs call this for critical results
- Signature (required by most institutions)
- Designation/specialty
3. Clinical Information
This is the most commonly underfilled section - yet it directly affects:
- Test selection and add-on testing by the lab
- Reference range interpretation (e.g., pregnancy, drug therapy)
- Prioritization by the laboratory
Include:
- Relevant diagnosis or differential (e.g., "?PE", "known CKD", "post-transplant")
- Relevant medications (especially anticoagulants, immunosuppressants, lithium)
- Relevant clinical context (e.g., "Day 3 post-op", "HIV positive", "pregnant - 28 weeks")
- Specific question being asked (e.g., "monitoring methotrexate toxicity", "?acute MI")
4. Specimen Details
- Type of specimen (blood, urine, CSF, sputum, tissue, swab site, etc.)
- Date and time of collection (critical for time-sensitive tests: troponin, cortisol, cultures)
- Collection conditions (fasting vs non-fasting, mid-stream vs catheter urine)
- Tube type used (gold/SST, purple/EDTA, blue/citrate, green/lithium heparin - must match test requirements)
5. Tests Requested
- Write test names clearly and specifically
- Use standardized abbreviations your lab recognizes
- For panels, state the panel name (e.g., "LFTs", "U&E", "TFTs", "FBC")
- For microbiology: specify organism suspected and antibiotics already given
- For histopathology: describe the specimen site, laterality, and clinical question
6. Priority / Urgency
- Routine - standard turnaround
- Urgent - faster processing, notify clinician
- Stat / Emergency - immediate processing, direct phone call with result
7. Date of Request
- Must be present for audit and medico-legal purposes
Common Investigation Categories - What to Write
Haematology
| Test | What to specify |
|---|---|
| FBC | Usually no extras needed |
| Coagulation (PT/APTT) | State if on anticoagulants |
| Blood film | Clinical indication (e.g., "?malaria", "haemolytic anaemia") |
| Group & Screen / Crossmatch | Number of units required, urgency |
Biochemistry
| Test | What to specify |
|---|---|
| U&E, Creatinine | eGFR auto-calculated; note if AKI |
| LFTs | Note hepatotoxic drugs |
| Troponin | Time of symptom onset (for serial interpretation) |
| TDM (drug levels) | Drug name, dose, timing of last dose, time of sample |
| Glucose | Fasting or random; note if diabetic |
| HbA1c | State if haemoglobinopathy known (affects method) |
| Cortisol | Time of sample; state if on steroids |
Microbiology
| Test | What to specify |
|---|---|
| Blood cultures | Aerobic + anaerobic; note if indwelling line |
| Urine M,C&S | Collection method (MSU/catheter/SPA) |
| Sputum culture | State if TB suspected (special handling) |
| Wound swab | Specify site precisely |
| CSF | Clinical diagnosis; request protein, glucose, cells, culture, virology as appropriate |
Imaging Requests
Additional elements required:
- Clinical question (e.g., "?pulmonary embolism", "?free air under diaphragm")
- Relevant allergy (especially contrast allergy for CT/MRI)
- Renal function (for iodinated contrast - eGFR required)
- Pregnancy status (for ionizing radiation)
- Previous relevant imaging (for comparison)
- Radiation dose justification (required by IRMER/Ionising Radiation regulations)
Histopathology / Cytology
- Exact site of biopsy/specimen (laterality, organ region)
- Type of specimen (core biopsy, excision, FNA, etc.)
- Clinical diagnosis or differential
- Relevant history (prior malignancy, immunosuppression, previous biopsies)
- Fixative used (formalin is standard; fresh tissue needed for some tests)
Common Errors to Avoid
| Error | Consequence |
|---|---|
| No clinical information | Test may be rejected; result interpretation impaired |
| Wrong tube type | Sample rejected; repeat venepuncture needed |
| Missing time of collection | Time-sensitive results uninterpretable |
| Illegible handwriting | Test not performed or wrong test done |
| No urgency stated | Routine processing of an acute sample |
| Incomplete patient ID | Result attributed to wrong patient |
| No anticoagulant history on coag screen | Misinterpreted result |
| Contrast allergy not noted on imaging | Anaphylaxis risk |
Practical Tips
- Call ahead for unusual tests or very urgent requests - don't rely on the form alone.
- Check local lab handbook for tube types, sample volumes, and opening hours for specialist tests.
- Fasting bloods: Lipids and glucose require a minimum 8-12 hour fast. Note "fasting" on the form.
- Timed samples: Mark exact time on form AND tube (cortisol, serial troponins, drug levels).
- Critical values: Ensure your bleep/phone number is on every form - labs are required to contact you directly for life-threatening results.
- Paired samples: Some tests require simultaneous samples (e.g., serum + urine osmolality, paired serum + CSF glucose).
A well-written investigation request is a clinical communication tool. The more context you provide, the more the laboratory, radiology, or pathology team can help you answer your clinical question accurately.
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