Tetracycline residues in organic and commercial eggs in public markets of Davao City, Philippines

The study was conducted in Davao City, the regional capital of the Davao Region (Region XI) in the southeastern part of Mindanao, Philippines. Covering approximately 2,443.61 square kilometers and home to 1.77 million people, the city serves as a major trading and food distribution hub in the region. The Davao Region contributed 5.43% of the country’s total chicken egg production of 583,234 metric tons, with Davao City accounting for 68% of the region’s output in 2019 (PSA, 2020). Given the high production and distribution volume, the use of antibiotics in poultry farming to prevent diseases, treat infections, and promote growth is inevitable. Such practices increase the potential for antibiotic residues, particularly tetracycline, to enter the local food supply chain, which is especially relevant in wet markets, where quality control may be limited.

A quantitative repeated cross-sectional design was used to determine tetracycline residue levels in organic and commercial chicken eggs sold in wet markets in Davao City. This design enabled the systematic detection of potential variations in residue concentrations across sampling intervals. Sampling was conducted in three rounds at one-week intervals within a single month to assess fluctuations in tetracycline levels and compliance with antibiotic withdrawal periods.

The egg samples were purchased from vendors across three representative public markets. Vendors were first interviewed to identify their suppliers and ensure the proper representation of local distribution. The sample size was calculated using OpenEpi (version 2.3.1) based on an assumed prevalence of 8% from the study by Yang et al. (2020) at the 95% confidence level. A total of 228 eggs were collected, comprising 114 commercial and 114 organic samples.

After collection, egg samples were stored at 4°C and transported in ice boxes to the Clinical Laboratory of the University of the Immaculate Conception. The samples were kept away from light to preserve the stability of residue. Egg yolks were separated and transferred individually into 50 mL centrifuge tubes for analysis following the protocols described in the Tetracycline ELISA Test Kit (A327256). For sample preparation, 1 g of homogenized egg was mixed with 3 mL of 2X sample dilution buffer, shaken for 20 min, and centrifuged at 4,000 rpm for 5 min. The resulting supernatant (100 μL) was used to test the enzyme activity. The reagents and wash buffers (1X and 2X dilutions) were prepared according to the manufacturer’s instructions. The assay procedure involved pipetting 100 μL of standards and prepared samples (in duplicate) into the designated wells of a 96-well microplate, followed by antibody addition and incubation at room temperature. After washing, 100 μL of TMB substrate was added, incubated for 15 min, and then stopped with 100 μL of stop solution. The absorbance was measured immediately at 450 nm using a BIOBASE BK-EL10A microplate reader. Seven calibration standards, along with blank and spiked egg samples, served as controls. The assay had a dilution factor of 4 and a detection limit of 0.4 ng/g for egg samples. The final concentrations were compared with the established Maximum Residue Levels to determine the frequency of samples beyond the threshold limit.

This study estimated consumers’ dietary exposure to tetracycline residues through egg consumption. The mean concentrations of tetracycline residues in eggs and average daily consumption based on 60 kg body weight for adults and 10 kg body weight for children were considered. ADI values of 0-30 ng/g/day for tetracycline residues, established by the Joint FAO/WHO Expert Committee on Food Additives (JECFA, 2002), were used to calculate the Hazard Quotient. Risk assessment was performed by calculating the Hazard Quotient (HQ) using the following formula:

EDI was calculated using the formula given by Juan et al. (2010).

An estimation of egg availability and consumption of eggs in the Philippines was carried out based on the report of the Department of Agriculture, Government of the Philippines.

The collected data were analyzed using JASP 0.19.3.0. Descriptive statistics were calculated to summarize tetracycline residue concentrations in both organic and commercial eggs, and the Mann-Whitney U test was used to examine whether there was a significant difference between the residue levels found in organic and commercial eggs.

Table 1 summarizes the tetracycline residue concentrations detected in eggs sampled from the public markets in Davao City. Organic eggs had a mean tetracycline concentration of 18.16±82.5 ng/g, whereas commercial eggs had a much lower mean concentration of 1.10±37.2 ng/g. The relatively large standard deviations, particularly in organic eggs, indicate pronounced variability in residue levels among individual samples, suggesting that while most eggs contained low or undetectable residues, a small number had markedly higher concentrations, inflating the dispersion around the mean. These values are considerably below the maximum residue limit (MRL) of 400 ng/g set by BPS (2015), indicating safety for consumer intake. The observed concentrations were consistent with the trends reported in previous studies. For example, a study in Egypt found tetracycline residues ranging from 18.73 to 265.9 ng/g in eggs from markets and farms (Amir et al., 2023). In contrast, another study found average levels of 3.10 ng/g, with a range of 0.56-9.77 ng/g (Kamali et al., 2020). Thus, eggs from both commercial and organic sources generally comply with international safety standards, despite the notable variability between individual samples and production systems.

These findings imply that eggs circulating in the public markets of Davao City, whether organic or commercial, pose minimal and immediate risk of exposure to tetracycline residue. Regulatory controls and production standards are effective in maintaining residual concentrations at safe levels. Nonetheless, continuous monitoring remains necessary, especially given the higher mean of organic samples, to maintain food safety and consumer confidence.

Organic eggs exhibited a mean tetracycline concentration of 18.16±82.5 ng/g, which was markedly higher than the mean value of 1.10±37.2 ng/g found in commercial eggs (p=0.004). These findings illustrate a substantial difference in the residue levels between the two egg types. This had a small to moderate effect size (r_pᵦ=−0.228), with Group 1 (commercial egg) tending to have lower values than Group 2 (organic egg). The narrow confidence interval suggests that this negative difference is reliable, and the true population effect is likely to fall between small (-0.081) and moderate (-0.364) negative associations. The overall trend follows previously reported data from both in local and international markets, tetracycline residues are frequently detectable in eggs, yet their levels are typically well below established safety thresholds, such as the maximum residue limit (MRL) of 400 ng/g set by BPS (2015).

Organic eggs exhibited significantly higher levels of tetracycline residues than commercial eggs, which may seem counterintuitive given the expectation that organic products should be “chemical-free.” This discrepancy could arise from several factors, including environmental exposure and cross-contamination, in which organic farms might share water, feed, or equipment with conventional farms, leading to indirect exposure (Sarkar et al., 2023; Sharma et al., 2018). Additionally, some organic certification standards permit limited therapeutic use of antibiotics under strict veterinary oversight, and failure to fully observe withdrawal periods could result in residual presence. Variability in supply chain practices and the less stringent enforcement of organic certification can further contribute to unexpected residues in organic eggs. Moreover, differences in farm management and health protocols might inadvertently expose organic hens to antibiotics, particularly if disease control options are limited. The wide variation in individual residue levels, reflected in the large standard deviation, underscores the complexity of the antibiotic residue dynamics in organic production systems. These findings highlight the importance of continuous surveillance, robust regulatory enforcement, and greater transparency in organic poultry production to maintain consumer trust and ensure food safety.

The health risks associated with dietary exposure to tetracycline residues from organic and commercial eggs in Davao City were evaluated by comparing the estimated daily intake (EDI) for adults and children with the acceptable daily intake (ADI) of 30 ng/kg/day (Table 2). All calculated EDIs (0.0002-0.0261 ng/kg/day) were several orders of magnitude lower than the ADI, yielding hazard quotients (HQs) between 7.5×10⁻⁶ and 1.2×10⁻⁴, which are far below the threshold value of 1 and therefore indicate no appreciable risk of adverse health effects from dietary exposure to tetracycline residues through consumption of eggs at current intake levels. These findings imply that, under the existing production and marketing conditions in Davao City, the current use patterns of tetracyclines are unlikely to pose a significant health concern for adult and child consumers, while still highlights the need for routine surveillance and prudent antibiotic stewardship to prevent residues from exceeding maximum residue limits and to limit the emergence of antimicrobial resistance (Bishnoi et al., 2025).