1. Nahar, Ludmila
  2. Brodell, Robert T.

Article Content


Chief complaint: Black spot on the right thumb.


A 37-year-old female patient presents with a 3-day history of a slightly tender, black spot on the right thumb. She reports minor trauma to this area 1 week prior. When it first appeared, the lesion appeared violaceous. She denies any history of recent ultraviolet radiation exposure. She also denies any personal or family history of melanoma.


* Prior treatment: A bandage was placed on the area after minor trauma. She was taking no medications.


* Prior biopsy: None.


* Skin history: Seborrheic keratosis on the back, removed by cryosurgery in 2018.


* No other significant laboratory/study findings




There is one image provided with this consult (Figure 1). The image shows a 5 x 2 mm jet black macule on the proximal phalanx of the right thumb. The lesion is symmetrical and has regular borders and uniform coloration.

Figure 1 - Click to enlarge in new windowFIGURE 1. Blood blister on the proximal phalanx of the right thumb. It has symmetrical macule, regular borders, and uniform coloration.


Fully satisfactory.




* Lesions: The finding of purpura on a thumb precipitated by minor trauma is typical of a blood blister.




* Skin care recommendations: None.


* Medication recommendations: None.




* Refer to dermatology clinic (face-to-face) to evaluate if the lesion does not fade over the next few weeks.



A solitary black macule at the site of minor trauma is typical of blood blister (Leatherman, 2004). The trauma breaks blood vessels leading to subcorneal collection of heme (blood) beneath the thick stratum corneum of the palm or sole. This should be distinguished from "pinch purpura," which is associated with systemic amyloidosis because normal supporting tissue is not present in surrounding vessels, leading to macular purpura from insignificant trauma. It is frequently present around the eyes, within the folds of the neck, in the axilla, beneath the breasts, and in the anogenital region (Metzger et al., 2019). Because the patient reported a history of minor trauma a few days before the lesion appeared and this is a solitary lesion, systemic amyloidosis can be ruled out. The differential diagnosis might include acral lentiginous melanoma (Figure 2). The melanoma is usually asymmetrical and has irregular borders, variegation in color, and a large size (>6 mm in diameter; Tsao et al., 2015).

Figure 2 - Click to enlarge in new windowFIGURE 2. Early melanoma on the back of the left hand shows ABCDE findings typical of the ABCDE mnemonic. (A) Asymmetry, (B) an irregular notched Border, (C) slight variegation of Color (tan, brown, and black); (D) large Diameter (larger than 4 mm, or larger than a pencil eraser; and, (E) Enlarged recently (E).



Leatherman M. (2004). What caused a "blood blister" to develop?. Advances in Skin & Wound Care, 17(7), 341. [Context Link]


Metzger Y. C., Cook D. L., Holmes T. E. (2019). Systemic amyloidosis. JAMA Dermatology, 155(3), 371. [Context Link]


Tsao H., Olazagasti J. M., Cordoro K. M., Brewer J. D., Taylor S. C., Bordeaux J. S., Chren M. M., Sober A. J., Tegeler C., Bhushan R., Begolka W. S. (2015). Early detection of melanoma: Reviewing the ABCDEs. Journal of the American Academy of Dermatology, 72(4), 717-723. [Context Link]


1 The standardized teledermatology reader report format is available for authors on the journal's Web site ( and on the submissions website online at [Context Link]