Study design
We conducted a randomized, double-blind clinical trial at two sites in Madrid (Centro Sanitario Sandoval and Hospital Clinico San Carlos, to compare the effect on immediate pain of PGB diluted with a local anaesthetic (mepivacaine hydrochloride 1%, MV) or PGB alone, and both groups either with a long 19G (long) or short 21G (short) IM needle. The study was performed during the first half of 2015. All patients signed informed consent. The study was approved by the local institutional review board (Ethical Committee of Clinical Research, Health Research Institute of the Hospital Clínico San Carlos) with approval number CEIC#14/376, and registered at the EU Clinical Trials Register with EudraCT Number: 2014-003969-24. The Consort flow diagram of the study is presented in Fig. 1.
Participants
Eligible subjects were 18 years of age or older diagnosed with primary syphilis; this diagnosis was based on both clinical and positive serological test results (non-treponemal and treponemal tests). Syphilis treatment required one IM dose of PGB 2400,000 million units (PGB 2,4 MU). We excluded patients with any sensory perception diagnosis or cognitive impairment unable to understand or evaluate the visual analog score on pain. Patients with previous penicillin allergy were also excluded.
Intervention
PGB was diluted with 6 mL of sterile water as indicated in information pamphlet (Benzetacil 2.400.00 U/6 mL, Reig Gofre, Barcelona, Spain); when MV was added to PGB, it was diluted with 5.5 mL of sterile water. A 0.5 mL dose of 1% MV was used to dilute with PGB; this corresponds to a total dose of 5 mg of MV. The study nursing team was trained to administer IM in the upper-outer quadrant of the buttock in the same way. Patients were randomly assigned to one of the four following groups: group 1, anesthetic/long 19G needle; group 2, anesthetic/short 21G needle; group 3, no anesthetic/long 19G needle; group 4, no anesthetic/short 21G needle.
Randomization and masking
The study participants were randomly assigned to study groups by a computer-generated randomized number system in blocks. The randomization was done 1:1:1:1 in blocks of four. The randomization sequence was hidden for the investigators who made the selection of patients. The design of the study was double-blind: neither the patient nor the investigator who analyzed the pain results knew the assigned group.
Outcomes
The primary objective of the study was the effect on local pain immediately after the injection; patients indicated the intensity of pain on a visual scale enumerated from 0 (no pain) to 10 (maximum pain) just after IM injection. Secondary objectives were pain intensity 6 and 24 h after the IM injection; this information was obtained by a phone call. Information was also collected on HIV status, age, sex, race, and body mass index (BMI).
Statistical analysis
Sample size calculation
In the absence of any published study regarding the effect of needle gauge on pain, we estimated the sample size with data from a convenience sample of consecutive patients with syphilis treated with one dose of IM PGB 2.4 MU. We studied immediate pain after the injection, in 22 patients distributed in the same four groups; we assessed needle gauge and the effect of anesthetic with a visual pain score (0-10). We compared 19G with anesthesia vs. 21G with anesthesia; 19G without anesthesia vs. 21G without anesthesia; and 19G with anesthesia vs. 19G without anesthesia. The estimation of the global sample size was based on the larger sample required for comparisons. Based on expected mean pain intensity of 7.6 in the 19G group without anesthesia and 8.6 the 21G group without anesthesia, with a significance level of 1.7% (Bonferroni adjustment 0.05/3) and statistical power of 80%, we considered that 104 patients (26 in each group) were needed.
Qualitative variables were summarized by their frequency distribution and quantitative variables by their mean and standard deviation (SD). Baseline participant characteristics were reported according to the randomly assigned intervention group with the use of descriptive statistics. To evaluate the differences in the intensity of the pain (immediate, 6 and 24 h after the injection) between the study groups, a multiple linear mixed model was fitted for controlling the hospital-cluster effect with adjustment for BMI. All analyses were based on intention to treat randomization status, and the statistical significance was assessed at the Bonferroni-corrected for multiple testing (α/3 = 0.017). All analyses were performed using STATA 12.0 software.