The 2020 National Survey of Drug Use and Health (NSDUH) estimates that almost 60% of people aged 12 or older used tobacco, alcohol, or an illicit drug in the past month (SAMHSA., 2021). Substance use and substance use disorders affect all segments of the population, including females of reproductive age. In NSDUH data, nearly two thirds of females >18 years of age reported alcohol use, one quarter reported nicotine use, and more than one sixth reported cannabis use in the past year (SAMHSA, 2021). Additionally, >3% reported opioid misuse (SAMHSA, 2019). The National Health and Nutrition Examination Survey estimates that 3% of women of reproductive age had past month prescription opioid use and 1.6% had at least one opioid prescription for 90 days or more, highlighting the prevalence of opioid use outside the context of an opioid use disorder (Elmore et al., 2021). The overdose rate for females of reproductive age increased by 38% between 2017 and 2020, primarily driven by synthetic opioids, and pregnancy associated overdose increased by 81% (Bruzelius and Martins, 2022).

Substance exposures are common, particularly in the context of unplanned pregnancies or delayed pregnancy awareness. Nearly half of all pregnancies are unplanned with even higher rates among individuals who are young, low income, or identified as Hispanic or Black (Branum and Ahrens, 2017). Substance use during pregnancy, including during early stages of fetal development before pregnancy recognition, is associated with adverse perinatal and neurodevelopmental outcomes (Board et al., 2023, Dejong et al., 2019).

Stigma, bias, and measurement errors make it difficult to accurately characterize exposures (e.g., dose and timing) during pregnancy and the effects on offspring outcomes. Maternal self-report may underestimate true exposure due to stigma, fear of involvement of child protective services, or long recall periods (Bandoli et al., 2023, El Marroun et al., 2011, Garg et al., 2016, Greyson et al., 2021, Oni et al., 2022). Urine is the most common biological matrix used to monitor substance exposure and validate self-report due to the ease of acquisition, familiarity, large sample volume, relatively high automation, and low assay costs (Wabuyele et al., 2018). However, primary limitations are the short half-life of analytes and limited ability to capture past or cumulative substance exposures (Wabuyele et al., 2018). Blood and saliva are also used; however, both also have short detection windows. Blood collection is invasive and requires trained personnel, and saliva has the potential for contamination (e.g. from smoke, food, etc.) (Wabuyele et al., 2018).

Analysis of substance panels in keratinized samples (i.e., hair and nail clippings) has gained scientific and clinical interest due to expanded detection windows and the non-invasive nature of sample collection. Cosmetic hair products, however, produce both false positive and false negative results, and uptake and retention of substance use analytes vary by hair texture (Kidwell et al., 2000), which significantly confounds findings by race/ethnicity. Moreover, culture-related views and practices may preclude collection of samples from indigenous and/or African American participants (Wright et al., 2018). These limitations highlight the need for exploration and validation of nail samples as an alternative to hair or other biological matrices.

Although not fully understood, it is hypothesized that analytes of substance use are incorporated into nails by a dual mechanism, which includes deposition into the root of the growing nail via the blood flow, and incorporation via the nail bed during growth from the lunula to the beginning of the free margin (Palmeri et al., 2000). Thus, the content of nail clippings integrates analytes of substance exposure over a relatively long period of time (Palmeri et al., 2000). Fingernails have a growth rate of about 3 mm/month with a regeneration time of 3–5 months; toenails grow 1.1 mm/month with a regeneration time of 8–16 months (Fleckman, 1985, Gupchup and Zatz, 1999, Lin et al., 2004, Palmeri et al., 2000). Because nails form at a constant rate, in contrast to the cyclic rates of hair growth (Cappelle et al., 2015), this matrix has the potential to span a significant portion of gestation. Several studies have established laboratory approaches to detecting substance exposure in nail samples (Cappelle et al., 2018, Kim et al., 2008, Shu et al., 2015). One of the largest studies to date (n=10,349) described laboratory methods for extraction and quantification of multiple drug classes in nail clipping samples (Shu et al., 2015). De-identified samples obtained from drug courts, child advocacy centers, and other locations were analyzed for a broad panel of substances. Results demonstrated feasibility of reliably analyzing polysubstance panels in nail clippings; however, validity, sensitivity, and specificity could not be derived without comparison with a ‘gold standard’ (such as self-reported measures or biomarkers in more established biological matrices).

Analytes of prenatal substance exposure in nail clippings are one measurement incorporated into the newly launched HEALthy Brain and Child Development (HBCD) study. HBCD is a longitudinal national cohort which will enroll 7500 pregnant people across the US and follow offspring development from birth through middle childhood (“HEALthy Brain and Child Development (HBCD) Study,” n.d.). HBCD is focusing on multi-modal longitudinal assessment of childhood neurodevelopment, including in the subset of children with prenatal substance exposures. While HBCD will evaluate a broad spectrum of exposures, the primary exposures of interest include alcohol, nicotine, opioids, and cannabinoids. To inform study design considerations and interpretation of biomarker testing in nail clippings in HBCD and other studies, a review of the existing literature is necessary. A previous review by Capelle (Cappelle et al., 2015) focused on the use of nails in the quantification of illicit and licit substances and provided a summary of the literature through 2014, including a review of the recommended techniques for sample preparation and detection methods. The current review expands on the evidence presented earlier by focusing on the use of nail samples to assess exposure to substances during prenatal development, includes a more recent analysis of the literature, and compares validity of nail clipping assays through comparison to other biospecimen assays or self-report. Thus, the primary objective of this systematic review was to identify and synthesize the data on the validity of alcohol, nicotine, opioids, and cannabinoids in fingernail or toenail clippings with respect to established self-reported assessment methods or other biological matrices. While this review focused on any human studies, emphasis was placed on pregnant individuals.