Patient Encounter 2

A 55-year-old female is admitted to the hospital with an intra-abdominal infection. During the patient interview, she states that she is allergic to aspirin, codeine, sulfa drugs, penicillin, levofloxacin, and vancomycin. The reactions are described as follows:

Aspirin: Easy bruising

Codeine: Upset stomach and itching after one dose

Sulfa drugs: Mild rash occurred 2 hours after taking a double strength trimethoprim-sulfamethoxazole tablet prescribed for a urinary tract infection

Penicillin: Rash, itching, and shortness of breath

Levofloxacin: Upset stomach

Vancomycin: Burning sensation when the drug was infused

Based on the patient's descriptions, which of the reactions represent an allergic or pseudoallergic reaction?

Which medications would you want to avoid based on this history?

What further questions would be useful to ask before developing a care plan?

If narcotic analgesics are required to treat this patient, what medication may be helpful as a pretreatment?

© Desensitization is a potentially life-threatening procedure, and requires continuous monitoring in a hospital setting, with suitable access to emergency treatment and intubation if required. It should only be undertaken under the direction of a physician with suitable training and experience. In such hands, desensitization may present less risk than treatment failure with a less effective alternative medication.

The possibility of readministering a suspected drug may be safely tested by gradual dose escalation in some cases, and there are certainly many more patients who are harmed by inappropriately withholding medications than there are those who suffer

significant harm from testing and desensitization.

Only type I IgE-mediated allergy may be treated by classical desensitization. De-sensitization may occur within hours to several weeks, unlike specific immunother-apy injections for inhalant allergy (i.e., "allergy shots" which may take months of therapy before a patient realizes any benefit and years to complete). The mechanism of drug desensitization is poorly understood, but produces temporary drug-specific tolerance of the offending drug. Any interruption of therapy of 24 hours or more requires full repeat desensitization, and abrupt significant increases of dosage have been reported to break through the tolerance with some drugs.

The process probably involves either: (a) cross-linking small subthreshold numbers of bound IgE molecules gradually depleting mast cells of their mediators, or (b) binding of the IgE by monomers or hapten-protein entities which cannot cross-link the antibody. The low doses used at the beginning of all protocols would provide small amounts of antigen, favoring these mechanisms. Both drug-specific IgE and IgG serum concentrations increase after successful desensitization, but skin test pos-

itivity generally decreases.

Oral and intravenous protocols are available for most drugs in this category, with the oral route producing somewhat milder reactions, but the intravenous route providing more precision in dosing. Intravenous administration can also be used in unresponsive patients where the oral route is not feasible. Protocols generally begin at about 1% of the therapeutic dose and increase in intervals defined by the patient s reaction and the distribution and metabolism of the drug itself. Half-log10 dose increases (about threefold) are often tolerated.

Penicillin desensitization is the most common drug desensitization protocol, and is required for penicillin-allergic patients when penicillin is clearly the only treatment option, for example when syphilis is present in pregnancy. Protocols have been adapted to most antibiotics. Tables 54-6 and 54-7 describe procedures for intravenous and oral penicillin desensitization.

Table 54-6 Protocol for Oral Penicillin Desensitization

PhenoxymethyJ Penicillin

Concentration

Volume

Dose

Cumulative

Step

(units/mU

imL}

(Units}

Dose {Units)

1

1,000

0.1

too

100

2

1,000

0.2

200

300

2

1000

0.4

400

700

4

1000

0.8

800

1500

5

1,000

1.6

lr600

3,100

5

1,000

3.2

3,200

6,300

7

lr000

6.4

6,400

12,700

S

1GrOOO

1,2

12r000

24,700

10,000

2.4

24j000

48700

10

10,000

4.3

4S.OOO

96,700

11

80,000

1.0

SOOQO

176,700

12

80,000

2,0

160,000

336,700

13

80,000

4.0

320,000

656,700

14

30.000

S.O

640,000

1296,700

Observe for 30 minutes

15

500,000

0,25

125,000

16

S00,000

0.5

250,000

17

500,000

1.0

500,000

16

500,000

2.25

1/125,000

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