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Shelf life penicillin testing reagents


70 year old female with a history of lymphedema and recurrent cellulitis in affected lower limb was tested to Prepen and penicillin G. Red pruritic rashes had developed on a "large area of her body" on the 2-4 th day on oral penicillin and cephalexin. She had severe abdominal pains from oral clindamycin but not after intravenous administration.
Prepen was negative even at intradermal full strength dose, whereas penicillin G was 5/5 wheal and flare at 1:10 and 6/6 at 10,000 units intradermally. Concerned about reliability of penicillin G testing solution which was prepared 3 years ago and frozen in individual vials at 10,000 units each.

My question is whether to withhold penicillin in my patient based on the non-standardized reagent for minor determinant testing that was positive only at 1:10 and full dose (10,000 u), and negative results with Prepen.


We sought the expertise of Dr. David Khan. His answer is below.

In general, a positive skin test to benzylpenicillin (penicillin G) ,which is one of the minor determinants, even in the absence of positivity to the major determinant (PRE-PEN®), would be considered a positive penicillin skin testing and avoidance of penicillin would be recommended. However the fact that the benzylpenicillin was frozen for several years makes it challenging to interpret the results. The stability of benzylpenicillin when frozen and then rethawed has not been well studied (see the 2012 Ask the Expert “Shelf Life of Dilutions of Penicillin G” included below). Since that time, a study by Garg et al. evaluated the stability of various penicillins including benzylpenicillin when frozen and it was stable at 7 days but this was the longest period studied.(1)  Another common source of potential false positive skin tests is when benzylpenicillin is diluted according to the manufacturer’s instructions with sterile water. Sterile water will cause irritant skin test reactions and thus when diluting benzylpenicillin for skin testing, saline should be used. Given the uncertainty of the stability of benzylpenicillin when frozen and rethawed for this extended period of time (3 years), I think it would be prudent to repeat the skin tests with a freshly diluted aliquot of benzylpenicillin G.

1.    Garg A, Chan D, Ambados F, Lwin E, Song Y, Garg S. Penicillin stability in prefilled syringes for the purpose of skin testing for drug allergy. J Allergy Clin Immunol Pract. 2015;3(4):599-601.

Ask the Expert 2012
A: There is no answer to your question in terms of the exact specifics expressed, but we do have information in the literature that can give us some insight into the potential degree of degradation when penicillin is diluted and stored in general.

The abstracts which I have copied below deal with this issue. Perhaps the most salient of these in regards to your question is the Neftel KA, et al., study in which they found that 38% of penicillin G in solution decays at 20 degrees C within 24 hours, 50% at 37 degrees C, and 66% within 3 hours at 56 degrees C.

The other abstracts look at different factors effecting the degradation of diluted and stored penicillin.

As noted, although we cannot answer your question specifically as stated, it is quite clear that penicillin does degrade when diluted and stored, even if refrigerated.

In addition, here is a link to the Pharmaceutical Partners of Canada information on penicillin G sodium for injection.

The link will pull up a PDF format document which gives a clear statement on the effect of reconstituting penicillin, stating that "reconstituted solutions are stable for 24 hours at controlled room temperature not exceeding 25 degrees centigrade for five days under refrigeration."

Unfortunately, I was not able to find any information regarding the effect of freezing and thawing on dilutions of penicillin, presumably because this would not normally be done clinically. My guess is, however, that this would adversely affect the stability.

In sum, this information taken as a whole indicates that there clearly is an effect of dilution on the stability of penicillin which could affect the sensitivity of skin tests (compared to using a freshly constituted preparation). However, we should remember that the allergens contained in penicillin for which we test are actually those of the degradation products (minor/major determinants). For the major determinant, of course, we have a commercially available preparation. The effect of the content of the minor determinants might actually increase with degradation related to storage. However, I could find no information on this.

Br J Haematol. 1983 Jun;54(2):255-60.
Penicillin-G degradation products inhibit in vitro granulopoiesis.
Neftel KA, Müller MR, Wälti M, Erni J, Gugler M, Arrenbrecht S.
38% of penicillin-G in solution decays at 20 degrees C within 24 h, 50% at 37 degrees C and 66% within 3 h at 56 degrees C. These degraded penicillin-G solutions strongly inhibit growth and maturation of granulocytic stem cells in vitro. Inhibitory concentrations are in the range obtainable with high dose penicillin therapy in vivo. No cytotoxicity of degraded penicillin solutions on bone marrow cells was seen over 24 h. It is suggested that penicillin-G degradation products are responsible for severe granulocytopenia observed after high dose penicillin-G therapy.

Lancet. 1982 May 1;1(8279):986-8.
Effect of storage of penicillin-G solutions on sensitisation to penicillin-G after intravenous administration.
Neftel KA, Wälti M, Spengler H, de Weck AL.
Intravenous administration of a total dose of more than 200 million IU of penicillin-G led to sensitisation of lymphocytes and formation of specific anti-penicilloyl antibodies of the IgG class. These effects were prevented when the penicillin solution used was freshly prepared and given as a bolus rather than as a slow infusion. The causative antigens seem to be related not to penicilloylated high molecular weight impurities in the penicillin preparations, but to the degradation and/or transformation products of penicillin-G that form when the drug is in solution even if only for a few hours, and not only at room temperature but also at 4 degrees C. Thus penicillin solutions should be freshly prepared and administered from vials containing less than 10 million IU so that bolus doses can be given.

Am J Hosp Pharm. 1976 Oct;33(10):1005-10.
Stability of several brands of ampicillin and penicillin V potassium oral liquids following reconstitution.
Jaffe JM, Certo NM, Pirakitikulr P, Colaizzi JL.
The stability-time profiles of the active ingredient of five generically equivalent brands of penicillin V potassium for oral solution, and of five generically equivalent brands of ampicillin for oral suspension, were studied. Three controlled conditions were employed-refrigerated, room and elevated temperature-and all the samples were assayed chemically for drug remaining at specific time intervals after reconstitution. The results showed that considerable variations in the initial concentrations of active component existed among the various ampicillin and penicillin products. In one penicillin product the official content requirement was not met. The data also showed that although the labels on each of the commercial penicillin products tested indicate that the reconstituted products may be stored in a refrigerator for 14 days without significant loss of potency, only one penicillin product still met 90% of label claim (minimal potency requirement of the United States Pharmacopeia for dry powder). All ampicillin products tested were stable when stored at the conditions recommended by the manufacturers, but the trihydrate forms exhibited greater stability than the anhydrous forms, probably because of the more rapid dissolution rate of anhydrous ampicillin.

Am J Hosp Pharm. 1979 Feb;36(2):205-8.
Stability of penicillin V potassium in unit dose oral syringes.
Grogan LJ, Jensen BK, Makoid MC, Baldwin JN.
The stability of reconstituted penicillin V potassium (PVK) when stored in 6-ml plastic oral syringes at various temperatures and protected from light was studied. One batch of PVK was reconstituted with distilled water according to manufacturer's directions (label claim: 125 mg/ml). Samples of 5 ml were stored in plastic oral syringes at 4 C, 25 C, 41 C, 60 C or 75 C and assayed spectrophotometrically and microbiogically at various times. From an initial concentration of 113% of label claim, PVK stored at 4 C (refrigerated) reached 90% of manufacturer's label claim in 11.5 days (95% confidence level). PVK stored at 25 C (room temperature) was unstable after storage for less than 37 hours. PVK degradation followed a first-order process. No significant difference was found between the spectrophotometric and microbiological assay (p less than 0.05). Manufacturer's stability data for storage of reconstituted PVK in the original bulk container should not be applied to PVK repackaged in plastic oral syringes. The pharmacy department developed guidelines designed to prevent the administration of subpotent PVK.

Phil Lieberman, M.D.

Hope this helps.
Patricia McNally, MD, FAAAAI