Stains & CD markers
Acid phosphatase

Editor-in-Chief: Debra L. Zynger, M.D.
Chunyu Cai, M.D., Ph.D.

Last author update: 22 January 2020
Last staff update: 17 June 2021

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PubMed Search: Acid phosphatase[title]stains[TIAB]

Chunyu Cai, M.D., Ph.D.
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Cite this page: Cai C. Acid phosphatase. website. Accessed February 23rd, 2024.
Definition / general
  • Acid phosphatase is not a single enzyme but rather a group of enzymes that hydrolyze and release phosphate group from different substrates
  • By definition, they function best in an acidic environment and are normally localized in the lysosomes (Mol Pathol 2002;55:65)
  • In muscle biopsies, the acid phosphatase stain is an enzyme histochemical stain that relies on endogenous acid phosphatase activity in the muscle specimen to hydrolyze the artificial naphthol AS-B1 phosphate substrate into naphthol, producing a brick red reaction product (Dubowitz: Muscle Biopsy: A Practical Approach, 4th Edition, 2013)
    • Therefore, the acid phosphatase stain must be performed on cryosections of snap-frozen fresh muscle tissue
  • Acid phosphatase should be differentiated from the antibody based immunohistochemical stains Prostatic Acid Phosphatase (PAP or PSAP) and Tartrate Resistant Acid Phosphatase (TRAP)
Essential features
  • In muscle biopsies, acid phosphatase stain is mainly used to highlight macrophages, red rimmed vacuoles in inclusion body myositis, lysosome storage disorders and other hereditary or acquired vacuolar myopathies associated with abnormal lysosomal activity
  • Nonspecific acid phosphatase(s)
  • Lysosomes
Uses by pathologists
Microscopic (histologic) description
  • Staining of lipofuscin in myofiber is punctate granular, predominantly subsarcolemmal (Dubowitz: Muscle Biopsy: A Practical Approach, 4th Edition, 2013)
  • Staining of lysosomes or lysosomal vacuoles in myofiber is punctate and predominantly sarcoplasmic; see disease specific references under Positive staining - disease section
  • Staining of degenerating myofibers and paraspinal myofibers is a weak diffuse blush in the sarcoplasm
  • Staining of macrophages is strong cytoplasmic
Microscopic (histologic) images

Contributed by Chunyu "Hunter" Cai, M.D., Ph.D.
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Adult onset acid maltase deficiency

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Central core myopathy

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Cystinosis distal myopathy

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Degenerating myofibers

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Hydroxychloroquine myopathy

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Inclusion body myositis

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Infant onset acid maltase deficiency

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Lupus myositis

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Normal muscle

Positive staining - normal
  • Stains lysosomes in myofibers, which are inconspicuous in normal fibers
  • Stains lipofuscin in muscle fibers, which are usually subsarcolemmally located and increase with age
  • Increased in paraspinal muscles (Muscle Nerve 2015;52:45)
Positive staining - disease
Negative staining
Board review style question #1
    Which of the following components in muscle fiber stains acid phosphatase?

  1. Lipid droplet
  2. Lysosome
  3. Mitochondria
  4. Myosin filament
  5. Sarcoplasmic reticulum
Board review style answer #1
B. Lysosome

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