Generic selectors
Exact matches only
Search in title
Search in content
Post Type Selectors
Filter by Categories
Book Review
Case Report
Case Series
CMAS‡ - Pancreas - EUS-FNA Cytopathology (PSC guidelines) S1:1 of 5
CMAS‡ - Pancreas - EUS-FNA Cytopathology (PSC guidelines) S1:3 of 5
CMAS‡ - Pancreas - EUS-FNA Cytopathology (PSC guidelines) S1:4 of 5
CMAS‡ - Pancreas -Sampling Techniques for Cytopathology (PSC guidelines) S1:2 of 5
CMAS‡ - Pancreas- EUS-FNA Cytopathology (PSC guidelines) S1:5 of 5
CytoJournal Monograph Related Review Series
CytoJournal Monograph Related Review Series (CMAS), Editorial
CytoJournal Monograph Related Review Series: Editorial
Cytojournal Quiz Case
Letter to Editor
Letter to the Editor
Letters to Editor
Methodology Article
Methodology Articles
Original Article
Pap Smear Collection and Preparation: Key Points
Quiz Case
Research Article
Review Article
Systematic Review and Meta Analysis
View Point
View/Download PDF

Translate this page into:

CytoJournal Monograph Related Review Series

Nuances of the Papanicolaou stain

Department of Pathology, Deenanath Mangeshkar Hospital and Research Centre, Pune, India
Department of Pathology, Government Medical College, Nagpur, Maharashtra, India

*Corresponding author: Dr. Meherbano M. Kamal, Professor, Department of Pathology, Government Medical College, Nagpur, Maharashtra, India.,

This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

How to cite this article: Sathawane P, Kamal MM, Deotale PR, Mankar H. Nuances of the Papanicolaou stain. CytoJournal 2022;19:43.


The impressive list of achievements of Dr. G. N. Papanicolaou and his tedious journey from normal to abnormal human cell includes the importance of wet fixation of cells and the development of the unique polychromatic Pap stain. The 5-dye Pap stain method evolved through 2 salient phases. The first being the development of wet fixation using alcohol-ether to enhance cellular transparency and the second phase saw the introduction of various cytoplasmic counterstaining methods using orange G and EA (light green, Bismarck brown, eosin) and phosphotungstic acid, facilitating the distinction of cell types. The specific characteristics of the staining method is, the cellular transparency combined with crisp nuclear staining, achieved through tailored cellular fixation and cytoplasmic staining using variable dye and pH combinations. With little modifications if any the Pap stain continues to be applied uniformly globally. However, institutional supply of dyes and chemicals from different companies make minor modifications, that remain consistent, an essential part of the staining protocol. This chapter describes the preparation and principles of various components of the stain that are being currently used in our department.


Papanicolaou stain
Pap stain
Wet fixation
Gill’s modified OG-6
Gill’s modified EA
Rehydration of Pap smears


Among the major achievements in the history of cytopathology, the eponymous Pap stain method formalized by Dr. George N. Papanicolaou in 1942 was foundational.[1,2,3] The PAP stain has been used all over the world since half a century for the staining of cervicovaginal smear. Many modifications have been published after George Papanicolaou described the original staining technique in 1942. The staining procedure varies with various staining protocols and methodologies used in different laboratories. The PAP stain is a polychromatic counterstaining method consisting of stains such as Orange G 6 (OG6) and modified eosin azure (EA). The strength of the Pap stain is such that it results in:

  1. Well-stained nuclear chromatin

  2. Differential polychromatic counterstaining of cytoplasm

  3. Cytoplasmic transparency.

To achieve this, proper fixation of the smear is one of the most important prerequisite factors.


The purpose of cytological fixatives is to maintain the cytomorphologic characteristics and diagnostically essential elements of the cell. Ethyl alcohol is the fixative specifically recommended for cytological preparations. Fixation coarsens the cell structures and sharpens nuclear chromatin pattern and its details. Commonly used cytological fixatives include wet fixatives and dry fixatives.


  1. 95% ethyl alcohol and ether in equal volume. Nowadays, it is not used as it is inflammable in nature and has a pungent odor

  2. 95% ethanol: Fixation for routine cytological smear

  3. 100% methanol

  4. 95% denatured alcohol

  5. 80% isopropanol

  6. 80% propanol.

The cytology laboratory of G.M.C, Nagpur, uses 95% ethanol as the fixative for routine cytological smears for both cervicovaginal and fine-needle aspiration cytology smears. The slides should be fixed immediately in the fixative solution, as even a slightest air drying of the smear can alter cytomorphological features causing diagnostic problems.[1] The smear should be fixed at least for 20–30 min to assure adequate fixation. However, prolonged fixation for several days or even weeks will not alter cellular features. Fixative solution along with the smear may be refrigerated in such situations to minimize the evaporation.


Dry or coating fixatives are major substitutes for wet fixatives in special situations like cancer detection camps or when transportation of smears from distant collection centers is required. These fixatives are available commercially and are either aerosol (applied by spraying) or liquid based (dropped over a smear). They are composed of an alcohol base, which fixes the cells and a wax-like substance that forms a thin protective coating over the smear. Any ordinary hair spray can be substituted as coating fixative.


  1. The Pap smear is prepared in the usual way and immediately fixed with “dry fix” spray; or a few drops of liquid based fixative solution are put on the smear

  2. While applying the spray, the bottle must be held at least 10–12 inches away from glass slide, which will prevent layering and hole formation

  3. The slide is then placed on a flat surface for a few minutes to allow the “dry fix” to dry

  4. The coating fixatives must be removed from the smear before staining. For this, slides are kept in 80% ethyl alcohol for 1–2 h. Sometimes, two changes of 80% alcohol may be required to remove the coating fixative completely.

If the coating fixative is not removed entirely before staining, it gives a “bubbling” effect to the smear which interferes with the diagnosis.[2]


Nuclear stain hematoxylin

Hematoxylin is the most widely used nuclear stain. It is commercially available as an amorphous brown-colored powder form, which is extracted from heartwood of Central America Logwood – “Haematoxylum campechianum.”

  • Hematoxylin itself is not a dye, it has to be oxidized to “hematin” which is actual staining component

  • The natural oxidation process or ripening of hematoxylin in aqueous solution takes place gradually over a period of time by keeping the stain bottle in the sunlight area at least for a month

  • The instant ripening can be achieved chemically by the addition of oxidizing agents such as sodium iodate or mercuric oxide

  • Hematoxylin stains the DNA and RNA of the cell

  • This stain can be used either progressively (Box 1) (without the use of acid alcohol differentiation) or regressively (Box 2)

  • In the bluing step (Box 3), the absorbance peak of hematoxylin is shifted visually resulting in the change of color from red to blue

  • Harris, Gill’s, and Mayer’s reagents are the most commonly used alum hematoxylin (Box 4) in cytology laboratory.

The cytology laboratory of G.M.C.H, Nagpur, uses the regressive method of staining using Harris and Gill’s hematoxylin followed by differentiation with 0.5% acid alcohol and bluing in the running tap water.

Harris hematoxylin: (For 1 L)

  • Hematoxylin powder 5 gm

  • (Yucca diagnostics, MERCK)

  • Absolute methanol/ethanol (solvent) 50 ml

  • Mercuric oxide (oxidizing agent) 2.5 gm

  • Aluminum ammonium sulfate (mordant) 100 gm

  • Glacial acetic acid* 40 ml

  • Distilled water 1000 ml

Note: *Glacial acetic acid is used as a stabilizer and slows down the oxidation process.


  1. Dissolve hematoxylin in alcohol

  2. Dissolve alum in water and bring to boil

  3. Add dissolved hematoxylin to alum solution and again bring to boil

  4. Remove the flask from heat and immediately add mercuric oxide

  5. Stir the solution until a dark purple color appears

  6. Cool the flask in water bath

  7. Filter and store in dark bottle in refrigerator.


  • Being a regressive stain, Harris hematoxylin over stains the nucleus and the excess stain is removed with the help of running tap water followed by differentiation using 0.5% acid alcohol

  • The decolorizing acid is then removed by keeping the slides in running tap water for bluing

  • Timing in acid alcohol is essential for the final appearance of the nuclear chromatin

  • If acid bath is inadequate, the contrast between the chromatin and the parachromatin is less and uptake of the counterstain is also lessened.

Gill’s hematoxylin[3]

Used for the procedure of PAP stain. For the preparation of 1 L of Gill’s hematoxylin, the following chemicals are combined in large conical flask in the given order. Stir the mixture for an hour manually or preferably on a magnetic stirrer at room temperature.


  1. Distilled water 730 ml

  2. Ethylene glycol 250 ml

  3. Hematoxylin 2 g

  4. Sodium iodate (oxidizing agent) 0.2gm

  5. Aluminum sulfate (mordant) 17.6 gm

  6. Glacial acetic acid* 20 ml

Note: *Glacial acetic acid stabilizes aluminum-hematin complex and slows down the oxidation of dye.

  • Gill’s hematoxylin can be prepared easily and rapidly

  • The unoxidized hematoxylin forms a metallic scum on the surface of the stain, which can interfere with the staining and precipitate on the slides

  • Although no surface or bottom precipitate is recovered, it is a good practice to filter the stain, before staining

  • Fresh hematoxylin should be added when the level in staining jar drops

  • Being a progressive stain, Gill’s hematoxylin stains the nucleus to the desired intensity and this is followed by bluing

  • The optimum time for nuclear staining is 1–1½ min

  • If Gill’s hematoxylin is used as a regressive stain, then the time of nuclear staining is increased up to 10–15 min followed by differentiation using acid alcohol (0.5 %) and then bluing in running tap water or in lithium carbonate bath.

OG-6 modified (Gill’s modified OG-6)

  • It is a cytoplasmic counterstain

  • In OG-6, OG signifies Orange G (“G” is an abbreviation for German word “gelb” which means yellow) and 6 denotes the concentration of phosphotungstic acid added

  • OG is an acidic protein dye, which can be combined with other protein dyes

  • Other variants of OG are OG-5 and OG-8

  • Modified OG is a Gill’s modification and is a combination of OG and phosphotungstic acid. It minimizes precipitation and reduces filtering and staining time

  • Gill’s modified OG is stable in solution and gives high-quality staining results.


  1. Orange G stain Powder* 5 g (HIMEDIA)

  2. Phosphotungstic acid** 1.5 gm

  3. Distilled water 50 ml

  4. Absolute alcohol 950 ml

  5. Glacial acetic acid*** 10 ml.

    1. Dissolve OG and phosphotungstic acid in distilled water, warm if necessary

    2. Add ethanol and glacial acetic acid and mix

    3. Store in a dark bottle, filter before use.

Note: *OG-6 stains mainly the keratin in the cytoplasm as brilliant orange, yellow, or brown (parakeratotic squamous carcinoma cells, erythrocytes, hemosiderin crystals, and asbestos bodies). *OG modified is stored in amber colored bottle. It is a supersaturated solution and hence a thick precipitate may be collected at the bottom of the staining jar which should not be filtered but gently stirred with glass rod before the slides are introduced in the staining jar. After staining about 2000 slides, the used stain is discarded and replaced with fresh stock solution. **Phosphotungstic acid maintains the pH of the stain to acidic level. It is a non-volatile acid which acidifies the solution and increases the uptake of OG. ***Acetic acid plays an important role in the uptake of the dye by the cytoplasm.[4]

EA modified (Gill’s modified EA)

  • It is also a cytoplasmic counterstain

  • EA-65 is a polychromatic stain and contains 3 dyes: Eosin Y, Light Green SF yellowish, and Bismarck Brown Y

  • The number of light green and eosin molecules must be adjusted relatively to one another to ensure the balanced staining

  • Eosin is purely an acidic dye and binds mainly to protein

  • EA is followed by a number which denotes proportion of the dyes. There are different formulations of this product (EA36, EA50, and EA65). EA-65 contains higher percentage of Green stain component, also referred to as “PAP EA65 enhanced green.”

  • In EA modified formula, Bismarck brown has been eliminated.[4] Although not numbered, Gill’s modified EA is similar to EA-36 and EA-50, but offers improved staining performance.

The main ingredients are

  1. Light Green* 2 g

  2. Eosin** (LOBA) 2 g

  3. Phosphotungstic acid*** 1 g

  4. Distill water 480 ml

  5. 95% Ethanol 500 ml

  6. Glacial acetic acid**** 20 ml

Dissolve light green, eosin, and phosphotungstic acid in distilled water separately. Then, mix all the three solutions together and add 95% ethanol and glacial acetic acid. Mix them thoroughly.

  • EA modified contains light green and eosin only

  • In EA modified formula, Bismarck brown has been deleted as it is considered to have no distinguishable color effect. Combination of Bismarck brown and phosphotungstic acid create precipitation, alters color overtime and reduces shelf life

*Light green stains metabolically active cells, that is, parabasal and intermediate cells, histiocytes, columnar cells, and malignant cells. **Eosin stains the mature squamous cells, erythrocytes, nuclei, and cilia. Red cells stain blue green if eosin is exhausted. ***Phosphotungstic acid is added to adjust the pH of the stains and helps optimize the color intensity. It selectively excludes eosin from cytoplasm of certain cell types and permits it to be stained by Light Green or Fast Green. Those sites remaining unstained by hematoxylin, OG, and Light Green are then stained by eosin.

****Glacial acetic acid is included in formula as it gives better cytoplasmic color differentiation and less background staining. It maintains required pH for optimal staining.

Acid alcohol

0.5% acid alcohol.


  1. Conc. HCL 5 ml

  2. 70% ethanol or rectified spirit 995 ml.

Bluing agent

Scott’s tap water substitute (STWS), dilute aqueous solution of ammonium hydroxide, and lithium carbonate are the most commonly used bluing agents. Tap water may serve as a bluing agent if the water pH is higher than 8.

STWS - pH 8.02

  1. Distilled water 1000 ml

  2. Magnesium sulfate MgSO4 10 gm

  3. Sodium bicarbonate NaHCO3 2 gm.

(20 g of MgSO47H2O (Epsom salt) can be used instead of 10 g of magnesium sulfate). Mix the ingredients in water and stir them well. STWS should be discarded after each round of staining.

Other bluing agents

  1. 3 ml liquid ammonia is added to 97 ml of 70% ethanol

  2. Dissolve 1.5 g of lithium carbonate in 100 ml of distilled water (stock solution) and add 30 drops of this solution to 1000 ml of distilled water.

Stain maintenance

Stain maintenance is necessary to remove contaminants, floaters and to restore the activity of the stain.

  • Filter the stain daily: It is recommended because washed off cells may be a source of contamination. Hematoxylin forms a scum and OG may precipitate into crystals

  • Change the staining solutions:

  1. Hematoxylin: It has a long shelf life. Replace with fresh stain after 1500 slides are stained

  2. OG modified: Replace after 2000 slides have been stained. This too has a long shelf life

  3. EA modified: It has a short shelf life and so replace it after 1500 slides have been stained

  4. Xylene: Filter through filter paper to remove contaminant cells

  5. Alcohol: One jar should be replaced daily.

Staining procedure: [Manual method followed in GMC, Nagpur. Figure 1]

From the fixative, the slides are passed through the following solutions

Manual staining procedure for Papanicolaou stain.
Figure 1:
Manual staining procedure for Papanicolaou stain.
Title Time
Running tap water 2–3 min
Gills hematoxylin 10–15 min
Running tap water 2–3 min
0.5% acid alcohol 1 Dip
Running tap water 3–5 min
95% ethanol 10 Dips
Orange G modified 2–3 Dips
95% ethanol 2–3 Dips
EA modified 3–5 Dips
95% ethanol 10 Dips
Absolute alcohol 10 Dips
Dry the slides Xylene 3–5 min

EA: Eosin azure

Mount the slides in dibutylphthalate polystyrene xylene (DPX).

Automated stainer (Method followed in GMC, Nagpur)

From the fixative, the slides are taken out and fixed in the slide carrier of the automatic robotic arm of the automatic stainer. The following program is followed.

Running tap water 1 min
Gills hematoxylin 15 min
Running tap water 5 min
0.5% acid alcohol 1 Dip
Running tap water 5 min
95% ethanol 1 min
Orange G modified 1 Dip
95% ethanol 1 Dip/1 min
EA modified 5 min
95% ethanol 1 Dip/1 min
Absolute alcohol 1 min
Xylene 2 min
Xylene 1 min

EA: Eosin azure


  1. Nuclear chromatin is stained as classical blue to purple in color

  2. RNA associated with DNA is delicately stained so that nucleoli display their characteristic eosinophilia

  3. Bar bodies are stained conspicuously

  4. Mucus and cell debris are minimally stained, thereby exhibiting an unobscuring background

  5. The first counterstain, OG-6 modified stains keratin a brilliant orange, yellow, or brown

  6. The second counter-stain EA-36stains the cytolasm of superficial cells (cornified) various shades of pink

  7. Cytoplasm of the intermediate cells (non-cornified) stains pale blue or green

  8. Cytoplasm of parabasal cells stains deep green

  9. Candida stains red

  10. Trichomonas vaginalis stains grey green.

Important factors for optimum staining

Excellence of the PAP’s staining depends largely on the delicate tints and transparency of the stained cells. The quality of the stained slides is dependent on the staining time, solubility and percentage of the dye concentration that is used in making nuclear and cytoplasmic stains. The following factors are useful in maintaining this desired quality.

  1. The smears should not be allowed to dry at any time/ between any steps

  2. Prolonged standings of the stained smears in the final grades of alcohols should be avoided as it leads to destaining of the cytoplasm

  3. The stains should be kept well covered as water absorption causes a dense opaque stain. Evaporation of the alcohol changes the color balance and causes precipitation of the stain

  4. Daily filtration of alcohol through a tight pad of cotton wool is recommended. This removes “free floating” cells or “floaters” thus minimizing cross-contamination

  5. After staining approximately 800 slides, alcohol should be filtered with Whatman filter paper 1. This can be done at the weekends

  6. Hematoxylin remains relatively constant in staining. Fresh stains should be added daily to replace stain loss due to evaporation

  7. Daily filtration of xylene should be done through Whatman filter paper 1. This removes the moisture and floaters

  8. The absolute alcohol and xylene of the last few steps must be fresh. All alcohol and xylene should be replaced by fresh if the following is noted

    1. A pale murky staining color

    2. Loss of sharp contrast and staining reaction of nuclear chromatin and cytoplasm

    3. Microscopic water droplets on the smear

  9. When manual staining is done, it is a good practice to blot the slides carrier on a thick pile of blotting paper for a few seconds. This will minimize transfer of stain and alcohol from one jar to another and also save the solution

  10. Shelf life of the stains may be increased by storing them in dark (amber) colored bottles when not in use and keeping the staining jar covered

  11. Purchasing readymade stain is time saving exercise but they are not cost effective. Furthermore, the composition and shelf life are unknown and hence the staining results are not optimal.


Occasionally, it is desirable to destain and restain poorly stained slides or when special stains are required. They can be destained and restained after the removal of coverslip as follows.[2]

  1. The coverslips are removed by soaking the slides in xylene till the coverslip falls off

  2. The slides are transferred into a jar containing absolute alcohol for a sufficiently long time (½ of h)

  3. The slides are then placed in 1% acid alcohol solution till the smear is completely colorless. It may require a few dips to 1 min or even longer, depending on the thickness of the smear

  4. The slides are gently and thoroughly washed in running tap water so that all the traces of acid is removed

  5. Then the slides are stained by routine PAP staining technique or any other desired stain.


The rehydration procedure may be used for air dried smears stored upto 72 hours before staining with HE and Pap.[6] It must, however, be noted that squamous cells may be restored to a considerable extent after rehydration. Whereas cells of the secretory type often suffer irreparable damage. The simplest technique of rehydration is as follows:

  1. Place the air-dried smears in 50% aqueous solution of glycerol for 3 min

  2. Rinse the slides in two changes of 95% alcohol and then stain the slides by routine PAP stain.[3]


  1. Place coverslips on clean blotting paper

  2. Remove the smear from xylol, drain off the excess xylol

  3. Place one or two drops of mounting medium DPX on the smear, invert the coverslip on the smear

  4. Apply gentle pressure, so that the mounting medium will spread uniformly and to remove air bubbles squeeze out excess mounting medium

  5. Using xylene moistened soft absorbent paper, wipe the edges of the slide clean

  6. Lay the slide flat till dry.


  1. Xylene at room temperature: Most commonly used solvent for coverslip removal. It dissolves DPX. Its turnaround time is more and takes 72–94 h for coverslip removal

  2. Xylene at 56 degree Celsius: Slides are kept in glass jar containing xylene and immersed inside water bath maintained at 56°C.[5] Heat fastens the melting of DPX

  3. Freezing: Fastest method of coverslip removal. Slides are kept in freezing chamber of domestic refrigerator at 0–4°C with coverslip facing downwards

  4. Petrol: Dissolves DPX. Plastic airtight jar with cap is used

  5. Diesel: Dissolves DPX. Plastic airtight jar with cap is used

  6. Liquid nitrogen: Not easily available at the setup

  7. Ultrasonic vibrations: May cause damage to the tissue section

  8. Scratching coverslip along with application of ice block: Causes damage to smear/tissue section on the slide.



LIST OF ABBREVIATIONS (In alphabetic order)

DNA - Deoxyribonucleic acid

DPX - dibutylphthalate polystyrenexylene

EA - Eosin Azure

GMC - Government Medical College, Nagpur

HIMEDIA - company name

LOBA - company name MERCK - company name

OG6 - Orange G 6 Pap – Papanicolaou

RNA - Ribonucleic acid

STWS - Scott’s tap water substitute


  1. . A new procedure for staining vaginal smears. Science. 1942;95:438-9.
    [CrossRef] [PubMed] [Google Scholar]
  2. , . Papanicolaou Revisited Leyden: Coulomb; .
    [Google Scholar]
  3. , , , , . Inception and development of the papanicolaou stain method. Acta Cytologica. 2017;61:266-80.
    [CrossRef] [PubMed] [Google Scholar]
  4. , , , , . Cytology Technical Manual Cytology Research Center (ICMR) Maulana Azad Medical College New Delhi.
    [Google Scholar]
  5. , , , , . Alternative rapid methods for coverslip removal: A comparative study. J Clin Diagn Res. 2019;13:EC01-2.
    [CrossRef] [Google Scholar]
  6. , , . Routine air drying of all smears prepared during fine needle aspiration and intraoperative cytology studies. An opportunity to practice a unified protocol offering the flexibility of choosing a variety of staining methods. Acta Cytol. 2001;45:60-8.
    [CrossRef] [PubMed] [Google Scholar]
Show Sections