The nail as an investigative tool in medicine: What a dermatologist ought to know

Introduction

Over the years, there is increasing interest in the study of the nail in health and disease. We know of its special structure and biological uses. It is also useful in diagnosis and as a marker of systemic disease. However, not many dermatologists know the amount of attention it receives outside our specialty.

This article focuses on the non dermatological relevance of this appendage. With developments in molecular biology and genetics, the nail is increasingly being seen as an ideal source of obtaining human specimens. It has attained the status of “a true window”, not just to disease, but also to the health status of an individual.

Methods and Results

For this review, information was collected by a PubMed search of articles published regarding the nondermatological uses of nail specimens. We used the key-words “nail clipping,” “nail DNA,” “nail diabetes mellitus,” “nail clipping oncology,” “nail forensic,” and “nail biometrics.” The searches yielded 82, 685, 437, 8, 122, and 2 indexed articles respectively, in English. These articles were retrieved and classified as case reports, review articles, and clinical trials. Information pertaining to nondermatologic applications of nails was collected. The final data was analyzed and is presented in a narrative fashion.

Why Use the Nail?

With the increasing popularity of screening programs, the need for appropriate human tissue specimens has increased. The specimen should afford adequate sensitivity and specificity in detecting what it is supposed to detect; it should involve low costs, collection should cause minimal discomfort to both patients and practitioners, and it should be easy to store and transport.^[1] The nail satisfies most of these criteria.

Conventionally, human blood and serum are commonly used for diagnosis; however, the importance of alternative tissue sources has increased over the years due to various reasons. Venous blood collection may prove difficult, especially in special populations, for large-scale programs, or for international collaborative investigations.^[2] Alternative tissue sources include card-based blood spots, buccal scrapes, hair samples and nail clippings as these are uniquely accessible as well as capable of delivering host DNA and other details. Nail as an alternative tissue sources has been found useful for genetic diagnosis as a part of screening procedures,^[3] diagnostic procedures,^[4],[5] assessment of adverse reactions,^[6] familial and population genetic profiling,^[7] and molecular autopsy studies.^[8],[9] In fact, for molecular autopsy studies, nail may be the only specimen which can be used for defining the cause of death or for clinical genetic information important for the surviving family.^[2] In addition to the advantages of adequate sensitivity and specificity, low cost, ease of retrieval, minimal discomfort upon retrieval, and acceptability to both patients and practitioners, nail specimens are also easy to collect, store, and transport.^[1]

However, because of a lack of awareness and proper processing techniques in routine laboratories, nail samples have not widely been used as diagnostic tools.

Nail as a keratinized matrix

Both nail and hair are keratinized matrices capable of continuous growth, and incorporate compounds within their structure. This property can be utilized in monitoring long-term consumption of alcohol or drugs.^[10] Nail specimens are found useful in toxicology and especially as an alternative to hair specimens.^[10] Hair analysis has been an established tool for drug testing, driving ability examination, detection of gestational drug exposure, criminal assault, and post-mortem toxicology.^[11] Correspondingly, our understanding of the mechanisms of incorporation of drugs into the hair matrix is advanced.^[12],[13] In contrast, literature on incorporation mechanisms in nails is sparse;^[11] nevertheless, we know the following mechanisms of drug incorporation in nails:^[1]

1. Nail matrix incorporation occurs during the formation of the nail plate via matrix blood flow. Thus, an incorporated drug would be detectable only when the nail grows enough to reach the free edge (10–18 weeks based on average nail growth rate)
2. Nail bed incorporation occurs during nail thickening. The nail bed contributes 21% of the nail thickness.^[14] A drug incorporated in this manner would be detectable in distal nail clippings much earlier (as early as 2–3 weeks)^{[15],[16],[17]}
3. Environmental contamination occurs mostly in the distal part, which explains the presence of materials, mostly xenobiotics, in the distal nail ^[18]
4. Sweat-mediated transport is responsible for rapid initial incorporation (within 24 hours) of drugs such as zolpidem.^[1] Drugs are eliminated through sweat channels depending on their molecular weight and hydrophilicity.^[19] Nails have a water content of 9–10% and drug diffusion through the nail bed can lead to early detection at the free edge.^[14],[20] Laufen et al. reported an uptake of fluconazole as early as 8 hours after intake.^[17] Similar diffusion for topically applied terbinafine has also been studied.^[21]

The kinetics of drug incorporation in the nails have been especially well worked out for zolpidem, a drug used for drug-facilitated sexual assault.^[10] A single dose administered has been found to be detectable in all fingernail clippings from as early as 24 h to as late as 3.5 months. In fact, even the time of intake can be inferred from the analysis of single fingernail clippings. Nail analysis could thus be an alternative as well as a complement to hair analysis in cases of suspected drug-facilitated sexual assault, and for monitoring of consumption behaviour.

[Table - 1] summarizes drugs routinely and reliably tested for, in nail specimens. This “nail biologic monitor” has been found useful in monitoring long-term exposure to drugs, micronutrients and xenobiotics; it can even help in temporal correlation with the supposed period of exposure.^[22]

Table 1: Various drugs incorporated in nail

Nail as a source of DNA

Fingernail material is an excellent source of germline DNA for genetic analyses in almost all clinical settings.^[37] Although the use of hair for this indication is well-known, there are practical problems, with hair specimens often being reported inferior for diagnostic use due to poorly detectable DNA. The Baylor SUDEP Tissue Donation Program (STOP) reported that hair samples were often received without follicles or that their integrity may be compromised by prior chemical processing with hair-care products.^[2] Fingernails are a more reliable source of autologous DNA of high-quality.^[37]

The specialized structure of fingernails (embodying DNA in keratinized cells) makes DNA extraction more complex than with fresh somatic cells; hence, well-defined protocols and reagents have been designed for lysing keratin.^[38] These protocols optimize the yield and quality of pure, intact DNA which has been found good enough even for demanding techniques such as next-generation sequencing for HLA typing.^[37] Advanced techniques such as the Prepfiler Forensic DNA Extraction kit can yield a mean of 1 mg high-quality DNA (range, 0.5 to 2.3 mg) from 20 mg nail material (1 to 10 pieces of fingernail clippings, a few millimetres wide only).^[37] DNA extracted from toenails ^[39] or fingernails ^[40] has been used for genotyping and identification of individuals in genetic epidemiology and forensic studies. Some of the indications for nail plate-derived DNA are summarized in [Table - 2].

Table 2: Applications of nail plate-derived DNA

How to Collect Nail Specimens

Collection of nail specimens is as simple as collection of fingernail trimmings or overhang of nail plates.^[2] Specimens can be collected on a plain sheet of paper (as done for mycology) or in sterile 1.5 mL microcentrifuge tubes for more demanding DNA analyses. An adequate sample consists of at least 1 week of untampered fingernail growth (assuming an average growth rate of 3 mm/month).^[2],[66] Except for daily hygiene, no additional nail cosmetic or nail treatment should have been done. Hands are thoroughly washed with soap and warm water and allowed to dry. Sterilized conventional metal nail clippers are used and whole nail trimmings are transferred into pre-labelled tubes or containers for transportation. These can easily be stored at room temperature until use. Depending on the analyses required, only one nail clipping may be collected (for serial detection of drugs)^[10] or ten nails (for DNA analyses) may be collected. For serial detection of drugs, ring fingernails are preferred (because of their medium growth rate) and serial collection from the same nail is advised.^[67]

[Table - 3] and [Table - 4] summarize the advantages and disadvantages of using nails as a specimen.

Table 3: Advantages of using nail as a specimen

Table 4: Disadvantages of nail specimens

Techniques Used to Examine Nail

As the nail has a unique structure, specialized techniques are required to examine and quantify specific components. Some such techniques used for nail analysis are summarized below.

• Laser-induced breakdown spectroscopy is used for the analysis of varied biological substrates such as bacteria,^[77],[78] teeth,^[29],[79] hair,^[80] bones ^[81] and fingernails.^[82] It employs a focused high-power, short-pulsed laser beam directed onto the nail surface.^[1] Based on the analysis of emission spectra from the surface, varying elements can be analyzed.
• High-performance liquid chromatography has been used for determination of drugs such as selective serotonin-reuptake inhibitors and serotonin–norepinephrine reuptake inhibitors in nail clippings.^[68] It has also been used as a speedy, simple and accurate technique in forensic toxicology for elucidating the cause of death or drug abuse
• Ultraperformance liquid chromatography–tandem mass spectrometry has been used to detect triclosan and triclocarban in nails.^[83] The collected nail clippings are digested with sodium hydroxide and chromatographic separation is performed with methanol. Target compounds are then determined by mass spectrometry
• Micro-PIXE (particle-induced X-ray emission) and micro-RBS (Rutherford back-scattering spectrometry) have been used to determine three-dimensional concentration maps of 18 elements in the human nail viz., major elements (C, N, and O), minor elements (P, S, Cl, K, and Ca), and trace elements (Fe, Mn, Zn, Ti, Na, Mg, Rb, Br, Sr, and Se)^[84]
• Hard X-ray micro-analysis has been used to examine arsenic distribution in nail-clippings.^[36] Nail clippings embedded in polyester resin and cut in cross-sectional slices are analyzed for arsenic concentration in different areas
• Synchrotron-based XRF (X-ray fluorescence) mapping has also been used to evaluate arsenic micro-distribution in toenail clippings.^[2],[85]

Clinical Indications for Use of Nail Specimens

The diagnostic use of nail specimens is well established for the following indications:

Nail in diabetes mellitus

Diabetes mellitus is a metabolic disease characterized by high blood sugar either due to insufficient insulin production or poor responsiveness.^[86] Various systemic pathologic alterations and metabolic events in diabetes mellitus are known to affect the nail unit structure and composition, and a nail sample can be a useful for clinical investigations.^{[87],[88],[89]} Documented techniques for the detection and monitoring of diabetes in the nail unit include:

• Estimation of glycated nail proteins has been found to reflect average blood glucose control over the previous 6–9 months.^[90],[91] An analysis is possible even on a 10 mg sample. The normal reference range for glycated nail protein is 0.55–3.60 μmol/g nail. In diabetics, the values are significantly higher (median, 4.07 μmol/g nail). Nail analysis could therefore be a simple alternative for diagnosing diabetes in persons from remote areas
• Changes in the molecular structure of human fingernail proteins in diabetic and nondiabetic specimens have been documented on the basis of their Fourier transform infrared spectroscopy spectra.^[91] It has been concluded that nail proteins of diabetics contain α-helical structure (including the presence of amide II bonds), whereas nails of nondiabetic patients do not have the amide II structures
• The dielectric properties of keratin–water system in diabetic and healthy fingernails have also been evaluated.^[92] It was reported that the dielectric measurements of human nail could be used for the detection of diabetes
• Nasli-Esfahani et al. carried out elemental analysis of nail as well as other biological samples (serum, scalp hair, urine and other body fluids) in diabetic patients.^[93] They concluded that scalp hair and nail are the best biological samples for trace element analysis in diabetics (especially Cr, Se, and Mn)
• Laser-induced breakdown spectroscopy (LIBS) has been used for analyzing diabetic and nondiabetic nails.^[1] It was concluded that LIBS spectra of fingernail can be a valid screening tool for diabetics in a large population with the advantages of quick measurement, broad elemental coverage, and low cost ^[1]
• Nail fold capillaroscopic (NFC) images were evaluated quantitatively as well as qualitatively in 145 children with Type 1 diabetes mellitus.^[90] Increases in the number as well as length of capillaries, presence of mega-capillaries and Raynaud's loops, and intense red background (possible neoangiogenesis) were the recorded findings. Longer disease duration correlated with an increase in the number of capillaries, disturbances in distribution, as well as the presence of abnormal capillaries. NFC is a noninvasive, painless and easily repeatable test allowing digital storage of images; it can be used as an effective monitoring tool in diabetics.^[90]

Role of nail clippings in oncology

Recent oncological research has focused on the primary prevention of cancer and identifying individuals at risk. It is known that trace elements have inhibitory as well as causative roles in oncogenesis. In addition, exposure to trace elements can be predetermined (e.g. arsenic) and may also be a modifiable risk-factor.^[91] Nail-clippings are therefore regarded as valuable biomarkers for such exposures.^[93],[94]

The FINBAR (Factors Influencing Barrett's Adenocarcinoma Relationship) study group attempted to correlate trace element status in toenails with the risk of Barrett's oesophagus and oesophageal adenocarcinoma.^[91] Toenail clippings from 638 participants and healthy controls were analyzed for eight trace elements. There was a two-fold higher risk of Barrett's oesophagus with high toenail zinc, a borderline significant, increased risk with higher cobalt levels, and no association with levels of chromium, cerium, mercury, and selenium.

The nail has been proposed as a more reliable biological meter for arsenic than serum because elevated levels would be maintained in the former for a longer time. Further, external contamination of nails with arsenic is much less extensive compared to that of hair.^[94] Long-term exposure to arsenic can lead to adverse health effects ^[95],[96] Lu et al. concluded that high arsenic exposure in humans promotes cancer initiation,^[97] though the exact mechanism of arsenic's role in carcinogenesis remains unknown.^[98],[99] Mechanisms responsible for arsenic accumulation in nails are poorly understood. The affinity of arsenic to sulphydryl groups of nail keratins may be responsible.^[100]

Forensic importance of nails

The nail plate is an important substrate for diagnosis in forensic science.^[101] Forensic casework routinely involves examination of fingernail scrapings and clippings for foreign DNA. In this scenario, both in-vivo and in-vitro analysis of nail specimens assumes significance. Though finger-nails may not be as useful as fingerprints for identification, in many cases broken fingernail plates have been used to associate a suspect with the victim by comparing nail ridge patterns.^[102]

Matte et al. reported that up to 19% of the general population may have foreign DNA beneath their fingernails, whereas foreign DNA may be detected in 33% of forensic fingernail samples.^[103] The normally present foreign DNA also tends not to persist for long. This needs to be taken into account by forensic analysts when providing an opinion on the relevance of foreign DNA under fingernails.

In forensic toxicology, reports abound on the usefulness of detecting drugs of abuse in nails. Brown et al. reported the utility of fingernail clippings in testing for levels of anabolic steroids in sportspersons with doping charges.^[104] Other reports include amphetamine-type stimulants, methadone, cocaine breakdown products, phenylalkylamine derivatives, and cannabinoids being detected.^{[105],[106],[107],[108],[109],[110],[111],[112],[113]} Further, ethyl glucuronide (EtG) has been put forward as a new biomarker in nails for alcohol consumption behavior.^[114]

Miscellaneous medical disorders

Apart from the diseases discussed above, nail clippings/biopsies have been useful in other diseases. The utility of the “nail window” into systemic diseases cannot be undermined.^[115]

Nail biopsies have been found useful in gout to detect urate crystals in subungual horn.^[116] Tirado-González et al. described subungual urate crystals extruded subclinically in some cases of gout.^[116] Nail biopsies taken to evaluate fungal elements showed urate crystals instead and the history subsequently confirmed gout. It was reported that there were no tophi noted in or near the nail field. Such crystals probably occur via exudation/transudation of fluids into the nail structure, offering a “nail window” into haematic or metabolic abnormalities. The authors concluded that the cytological and histological findings in nail specimens could be used to evaluate nail diseases as well as systemic diseases.^[116]

Similarly, toenail nicotine levels have been used as biomarkers to predict the risk of coronary heart disease (CHD). In a nested case-control study involving 62,641 women followed up over 16 years,^[117] a statistically significant, dose-response association was seen between increased toenail nicotine levels and risk of CHD. The authors concluded that toenail nicotine levels are predictive of CHD among women independent of other risk factors.

Nails for biometrics

With advances in information technology, security aspects have become paramount. Authentication is a prerequisite for security with biometric authentication being an important mode. This involves automated recognition of individuals based on their physiological characteristics, identifying a person based on “who she/he is” rather than “what she/he has” (card, token, key); or “what she/he knows” (password, pin).^[119] Common characteristics used for biometrics include face recognition, fingerprints, handwriting, hand geometry, iris, vein, voice or retinal scan. The use of fingernail patterns as biometric markers has been evaluated and found to be useful.^[118] Herein, authentication is based on unique individual ridge patterns of the nail bed reflected on the nail plate surface, which can be evaluated by computational analysis even in low-resolution images.^[120] This has proved to be a very unique and stable biometric identifier, good enough for forensic as well as civilian applications.^[120] Extensive experimentation has validated its use.

The low-resolution nail plate images are acquired with a contactless, unconstrained imaging setup analyzing texture-based feature descriptors. Then, computational analysis is used for integrating nail plates from three fingers. Outcomes of rigorous experimental analysis on 2700 nail plate images found this to be a promising biometric modality.^[121] Nail bed pattern can also be analyzed with a laser-based broadband interferometer technique.^[122] Another system measuring spacing of the capillary loops with highly monochromatic light has also been evolved.^[123]

Hand-based biometrics has high user acceptance and reliability. Of these, the fingernail plate is characterized by high individuality,^[124],[125] with a high degree of distinctiveness, even among identical twins.^[126] Moreover, the hardened nail resists environmental effects, barring changes caused by nail diseases/disorders and malnutrition.^[127] This ensures high reproducibility as well.

Radiation dosimetry

Rapid and accurate determination of individual radiation exposure would be needed to screen exposed populations in case of a radiological/nuclear event. It has been seen that estimating the chemical or physical alterations produced in biomaterials can be used to determine the level of exposure. Radiation sensitivity of nails is relatively high and changes produced in nails exposed to radiation have been found to be a useful biodosimetry method. In addition, the radicals generated in condensed nail protein are stable over time.^[128],[129] An ex-vivo estimation of severity of radiation exposure based on the electron paramagnetic resonance nail dosimetry was evaluated by He et al.^[128] Human nail clippings were used to evaluate stable radiation-induced signal. It was found that a reliable triage based on radiation dosage was possible.^[130] The technique also ensured immediate and rapid dose assessment.

This review has some limitations. The topic being vast, we may have missed additional diagnostic applications of the nail unit not adequately represented in the indexed literature. In addition, with expanding developments in the field, a compendium such as this may fall short of the latest information at times despite our efforts to make it up-to-date.

Conclusion

It is clear from the recent growth in literature that the nail is not just an inert skin appendage, but. a dynamic part of the human body, reflective of the changes in the metabolic and genetic milieu. Nail specimens are a valuable diagnostic tool as they are easy to retrieve, without causing significant discomfort. The coming years are likely to see more research and expansion of knowledge in this field.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.