Suspected pulmonary embolism in a postoperative patient with renal impairment and previous contrast anaphylaxis

Preferred: ventilation–perfusion SPECT/CT (V/Q SPECT), with the low-dose CT used only for attenuation correction and anatomical localisation.

Reasoning through the competing constraints:

• No iodinated contrast, so it avoids the risk of a breakthrough reaction in a patient with documented anaphylaxis and protects a residual eGFR of 27.
• Accuracy: V/Q SPECT has a sensitivity and specificity broadly comparable to CTPA, with a much lower non-diagnostic rate than planar V/Q. It is a defensible first-line test in a haemodynamically stable patient.
• Logistics: a 22-gauge cannula is adequate for a 99mTc-MAA injection; the department closes in 3 hours but the study takes about 30 minutes of camera time.
• Dose: lower breast dose than CTPA; the CT component adds roughly 1–2 mSv.

Weigh against it: V/Q cannot diagnose alternative causes of breathlessness (pneumonia, aortic dissection), although the clear chest film and the absence of dissection-type pain make these less likely.

If V/Q is unavailable or the patient becomes unstable, CTPA with premedication and a resuscitation team present is acceptable. Because he is only 4 days post-laparotomy, a definitive answer matters: anticoagulation carries real bleeding risk, so an equivocal strategy is worse than a good-quality scan.

No - premedication lowers, but does not abolish, the risk of a severe reaction, and it cannot be allowed to substitute for choosing a contrast-free test.

• Standard regimens (for example prednisone 50 mg orally at 13, 7 and 1 hour before contrast, plus an antihistamine at 1 hour) reduce the frequency of mild reactions, but breakthrough reactions still occur in a few percent of high-risk patients and can be severe. There is no regimen with a zero failure rate.
• Premedication does nothing for non-IgE-mediated or vagal reactions, and does not protect renal function.
• The 13-hour schedule is impractical here: the patient is hypoxic and tachycardic with a high pre-test probability, so delay itself is a hazard.
• If CTPA is unavoidable, give a low-osmolar non-ionic agent in a resuscitation area with immediate access to adrenaline, and do not treat the premedication as a guarantee.

Document the historical reaction precisely and arrange allergy clinic review after discharge - the label "contrast allergy" is often applied loosely and drives lifelong avoidance of necessary studies.

First, ask why it is non-diagnostic - patient movement, poor inhalation phase, suboptimal injection timing, or genuinely equivocal mismatched defects. Review the images with the nuclear medicine physician; SPECT with CT correlation often converts a planar "non-diagnostic" into a confident report.

If it remains non-diagnostic and clinical probability is high:

1. Start therapeutic anticoagulation unless there is a strong contraindication; discuss with the operating surgeon given the recent laparotomy. Empiric treatment while arranging a definitive test is standard practice in high-probability patients.
2. Perform bilateral leg ultrasound. A proximal DVT confirms the need for treatment, but a negative study does not exclude pulmonary embolism.
3. Proceed to CTPA with premedication and resuscitation standby, or transfer to a centre with a working SPECT service - whichever is faster and safer for this patient.

Catheter pulmonary angiography is reserved for the rare case where both CT and V/Q are genuinely contraindicated and the result will change management. MR pulmonary angiography is not yet a reliable first-line alternative. Do not simply repeat the V/Q the same day, and do not leave a high-probability patient untreated while waiting.