To the Editor: Human immunodeficiency virus (HIV) transmission among injecting drug users (IDUs) is commonly attributed to the practice of sharing contaminated needles and syringes(1). The minimal infectious dose of HIV is unknown, but the volume of innoculum and quantity of virus in an exposure are believed to influence the risk of transmission(2-5). Both factors may be affected by the type of syringe used by an IDU. As an ethnographer(1989-92) and IDU (1968-87), one author (WZ) observed a change in syringes used by IDUs in Texas during the HIV epidemic. Syringes in current use have permanently attached needles and retain visibility less fluid and blood than syringes with detachable needles used previously.
Prior studies(6-8) have quantified the amount of blood in needle and syringe sharing simulations; the type of needle, detachable or integral cannula (permanently attached), was not specified. Only one study(8) incorporated the practice, common among IDUs, of rinsing with water between uses. We measured differences in fluid and blood retained in needle and syringe combinations in a series of experiments.
Four needle and syringe combinations shown inTable 1 were evaluated. To determine the volume of fluid retained, 20 syringes of each type were weighed using an electronic P160N Mettler balance. Each syringe with needle was weighed new and dry, and filled with 1 ml of deionized water. Water was expelled by fully depressing the plunger, and each syringe was reweighed. To simulate intravenous drug injection and syringe rinsing, 0.5 ml of 1× phosphate-buffered saline (PBS/drug solution) was drawn into each syringe. Air was expelled for a final volume of 0.4 ml. The needle was inserted into a tube of human blood of a known red blood cell count with ethylene diaminetetra-acetic acid (EDTA) anticoagulant and the plunger was drawn back 0.1 ml (registering/checking that the needle was in the vein). Contents of the syringe were expelled (injection), and an additional 0.1 ml of blood was drawn into the syringe and expelled(booting/flushing). The syringe was then rinsed twice with 0.5 ml of fresh PBS and the second rinse collected. The experiment was repeated with 10 syringes of each type. Manual red blood cell counts were performed on each final rinse using an improved Neubauer hemocytometer. Statistical analysis was performed using single-factor analysis of variance.
With the plunger fully depressed, syringes with detachable needles retained over 40 times as much fluid as integral cannula syringes. In simulations using whole blood and two PBS rinses, syringes with detachable needles retained a minimum of 300 times as much blood as integral cannula syringes. Table 2 presents mean volumes of fluid and blood retained in each needle and syringe combination.
To our knowledge, no one has addressed the effect of syringe type on HIV transmission among IDUs. If the volume of inoculum and the quantity of virus in an exposure affect the probability of infection, then integral cannula syringes are a less efficient means of transmission and thus a safer instrument for drug injection. Needle exchanges should be encouraged to distribute integral cannula syringes only and IDUs using syringes with detachable needles should be warned of the higher risks.
*William A. Zule
†Kathleen M. Ticknor-Stellato
*David P. Desmond
*Kenneth N. Vogtsberger
*Department of Psychiatry University of Texas Health Science Center at San Antonio†Department of Psychiatry University Hospital San Antonio, Texas
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